Support parameterized metrics

Metrics like Counter1 and Timer1 can now be given a field to
dynamically allocate submetrics for specific values.

Rename the versions with no field Counter0 and Timer0 to
indicate they do allocate submetrics.

Support Counter1-3 and Timer1-3 for up to 3 dimensional
submetrics of either kind.

Description.setFieldOrdering() can be used to control how
the submetrics are named in DropWizard. The default appends
field values to create a new submetric. Another option is
offered to insert field values before the last '/', allowing
things to be broken down by a slightly different layout.

Change-Id: Id2b4f59eb603eb8c8690753650a5720fc26d617b

gerrit-server/src/main/java/com/google/gerrit/metrics/CallbackMetric.java

@@ -20,24 +20,8 @@ import com.google.gerrit.extensions.registration.RegistrationHandle;
 /**
  * Metric whose value is supplied when the trigger is invoked.
  *
- * <pre>
- *   CallbackMetric<Long> hits = metricMaker.newCallbackMetric("hits", ...);
- *   CallbackMetric<Long> total = metricMaker.newCallbackMetric("total", ...);
- *   metricMaker.newTrigger(hits, total, new Runnable() {
- *     public void run() {
- *       hits.set(1);
- *       total.set(5);
- *     }
- *   });
- * </pre>
- *
+ * @see CallbackMetric0
  * @param <V> type of the metric value, typically Integer or Long.
  */
-public abstract class CallbackMetric<V> implements RegistrationHandle {
-  /**
-   * Supply the current value of the metric.
-   *
-   * @param value current value.
-   */
-  public abstract void set(V value);
+public interface CallbackMetric<V> extends RegistrationHandle {
 }

gerrit-server/src/main/java/com/google/gerrit/metrics/CallbackMetric0.java

@@ -0,0 +1,40 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+/**
+ * Metric whose value is supplied when the trigger is invoked.
+ *
+ * <pre>
+ *   CallbackMetric0<Long> hits = metricMaker.newCallbackMetric("hits", ...);
+ *   CallbackMetric0<Long> total = metricMaker.newCallbackMetric("total", ...);
+ *   metricMaker.newTrigger(hits, total, new Runnable() {
+ *     public void run() {
+ *       hits.set(1);
+ *       total.set(5);
+ *     }
+ *   });
+ * </pre>
+ *
+ * @param <V> type of the metric value, typically Integer or Long.
+ */
+public abstract class CallbackMetric0<V> implements CallbackMetric<V> {
+  /**
+   * Supply the current value of the metric.
+   *
+   * @param value current value.
+   */
+  public abstract void set(V value);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/Counter0.java

@@ -26,7 +26,7 @@ import com.google.gerrit.extensions.registration.RegistrationHandle;
  * For an instantaneous read of a value that can change over time
  * (e.g. "memory in use") use a {@link CallbackMetric}.
  */
-public abstract class Counter implements RegistrationHandle {
+public abstract class Counter0 implements RegistrationHandle {
   /** Increment the counter by one event. */
   public void increment() {
     incrementBy(1);

gerrit-server/src/main/java/com/google/gerrit/metrics/Counter1.java

@@ -0,0 +1,44 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+/**
+ * Metric whose value increments during the life of the process.
+ * <p>
+ * Suitable uses are "total requests handled", "bytes sent", etc.
+ * Use {@link Description#setRate()} to suggest the monitoring system
+ * should also track the rate of increments if this is of interest.
+ * <p>
+ * For an instantaneous read of a value that can change over time
+ * (e.g. "memory in use") use a {@link CallbackMetric}.
+ *
+ * @param <F1> type of the field.
+ */
+public abstract class Counter1<F1> implements RegistrationHandle {
+  /** Increment the counter by one event. */
+  public void increment(F1 field1) {
+    incrementBy(field1, 1);
+  }
+
+  /**
+   * Increment the counter by a specified amount.
+   *
+   * @param field1 bucket to increment.
+   * @param value value to increment by, must be >= 0.
+   */
+  public abstract void incrementBy(F1 field1, long value);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/Counter2.java

@@ -0,0 +1,46 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+/**
+ * Metric whose value increments during the life of the process.
+ * <p>
+ * Suitable uses are "total requests handled", "bytes sent", etc.
+ * Use {@link Description#setRate()} to suggest the monitoring system
+ * should also track the rate of increments if this is of interest.
+ * <p>
+ * For an instantaneous read of a value that can change over time
+ * (e.g. "memory in use") use a {@link CallbackMetric}.
+ *
+ * @param <F1> type of the field.
+ * @param <F2> type of the field.
+ */
+public abstract class Counter2<F1, F2> implements RegistrationHandle {
+  /** Increment the counter by one event. */
+  public void increment(F1 field1, F2 field2) {
+    incrementBy(field1, field2, 1);
+  }
+
+  /**
+   * Increment the counter by a specified amount.
+   *
+   * @param field1 bucket to increment.
+   * @param field2 bucket to increment.
+   * @param value value to increment by, must be >= 0.
+   */
+  public abstract void incrementBy(F1 field1, F2 field2, long value);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/Counter3.java

@@ -0,0 +1,48 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+/**
+ * Metric whose value increments during the life of the process.
+ * <p>
+ * Suitable uses are "total requests handled", "bytes sent", etc.
+ * Use {@link Description#setRate()} to suggest the monitoring system
+ * should also track the rate of increments if this is of interest.
+ * <p>
+ * For an instantaneous read of a value that can change over time
+ * (e.g. "memory in use") use a {@link CallbackMetric}.
+ *
+ * @param <F1> type of the field.
+ * @param <F2> type of the field.
+ * @param <F3> type of the field.
+ */
+public abstract class Counter3<F1, F2, F3> implements RegistrationHandle {
+  /** Increment the counter by one event. */
+  public void increment(F1 field1, F2 field2, F3 field3) {
+    incrementBy(field1, field2, field3, 1);
+  }
+
+  /**
+   * Increment the counter by a specified amount.
+   *
+   * @param field1 bucket to increment.
+   * @param field2 bucket to increment.
+   * @param field3 bucket to increment.
+   * @param value value to increment by, must be >= 0.
+   */
+  public abstract void incrementBy(F1 field1, F2 field2, F3 field3, long value);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/Description.java

@@ -28,6 +28,7 @@ public class Description {
   public static final String CUMULATIVE = "CUMULATIVE";
   public static final String RATE = "RATE";
   public static final String GAUGE = "GAUGE";
+  public static final String FIELD_ORDERING = "FIELD_ORDERING";
   public static final String TRUE_VALUE = "1";
 
   public static class Units {
@@ -42,6 +43,19 @@ public class Description {
     }
   }
 
+  public static enum FieldOrdering {
+    /** Default ordering places fields at end of the parent metric name. */
+    AT_END,
+
+    /**
+     * Splits the metric name by inserting field values before the last '/' in
+     * the metric name. For example {@code "plugins/replication/push_latency"}
+     * with a {@code Field.ofString("remote")} will create submetrics named
+     * {@code "plugins/replication/some-server/push_latency"}.
+     */
+    PREFIX_FIELDS_BASENAME;
+  }
+
   private final Map<String, String> annotations;
 
   /**
@@ -65,7 +79,7 @@ public class Description {
   /**
    * Indicates the metric may be usefully interpreted as a count over short
    * periods of time, such as request arrival rate. May only be applied to a
-   * {@link Counter}.
+   * {@link Counter0}.
    */
   public Description setRate() {
     annotations.put(RATE, TRUE_VALUE);
@@ -83,7 +97,7 @@ public class Description {
 
   /**
    * Indicates the metric accumulates over the lifespan of the process. A
-   * {@link Counter} like total requests handled accumulates over the process
+   * {@link Counter0} like total requests handled accumulates over the process
    * and should be {@code setCumulative()}.
    */
   public Description setCumulative() {
@@ -91,6 +105,12 @@ public class Description {
     return this;
   }
 
+  /** Configure how fields are ordered into submetric names. */
+  public Description setFieldOrdering(FieldOrdering ordering) {
+    annotations.put(FIELD_ORDERING, ordering.name());
+    return this;
+  }
+
   /** True if the metric may be interpreted as a rate over time. */
   public boolean isRate() {
     return TRUE_VALUE.equals(annotations.get(RATE));
@@ -106,6 +126,12 @@ public class Description {
     return TRUE_VALUE.equals(annotations.get(CUMULATIVE));
   }
 
+  /** Get the suggested field ordering. */
+  public FieldOrdering getFieldOrdering() {
+    String o = annotations.get(FIELD_ORDERING);
+    return o != null ? FieldOrdering.valueOf(o) : FieldOrdering.AT_END;
+  }
+
   /**
    * Decode the unit as a unit of time.
    *

gerrit-server/src/main/java/com/google/gerrit/metrics/Field.java

@@ -0,0 +1,136 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import com.google.common.base.Function;
+import com.google.common.base.Functions;
+
+/** Describes a bucketing field used by a metric. */
+public class Field<T> {
+  /** Break down metrics by boolean true/false. */
+  public static Field<Boolean> ofBoolean(String name) {
+    return ofBoolean(name, null);
+  }
+
+  /** Break down metrics by boolean true/false. */
+  public static Field<Boolean> ofBoolean(String name, String description) {
+    return new Field<>(name, Boolean.class, description);
+  }
+
+  /** Break down metrics by cases of an enum. */
+  public static <E extends Enum<E>> Field<E> ofEnum(Class<E> enumType,
+      String name) {
+    return ofEnum(enumType, name, null);
+  }
+
+  /** Break down metrics by cases of an enum. */
+  public static <E extends Enum<E>> Field<E> ofEnum(Class<E> enumType,
+      String name, String description) {
+    return new Field<>(name, enumType, description);
+  }
+
+  /**
+   * Break down metrics by string.
+   * <p>
+   * Each unique string will allocate a new submetric. <b>Do not use user
+   * content as a field value</b> as field values are never reclaimed.
+   */
+  public static Field<String> ofString(String name) {
+    return ofString(name, null);
+  }
+
+  /**
+   * Break down metrics by string.
+   * <p>
+   * Each unique string will allocate a new submetric. <b>Do not use user
+   * content as a field value</b> as field values are never reclaimed.
+   */
+  public static Field<String> ofString(String name, String description) {
+    return new Field<>(name, String.class, description);
+  }
+
+  /**
+   * Break down metrics by integer.
+   * <p>
+   * Each unique integer will allocate a new submetric. <b>Do not use user
+   * content as a field value</b> as field values are never reclaimed.
+   */
+  public static Field<Integer> ofInteger(String name) {
+    return ofInteger(name, null);
+  }
+
+  /**
+   * Break down metrics by integer.
+   * <p>
+   * Each unique integer will allocate a new submetric. <b>Do not use user
+   * content as a field value</b> as field values are never reclaimed.
+   */
+  public static Field<Integer> ofInteger(String name, String description) {
+    return new Field<>(name, Integer.class, description);
+  }
+
+  private final String name;
+  private final Class<T> keyType;
+  private final Function<T, String> formatter;
+  private final String description;
+
+  private Field(String name, Class<T> keyType, String description) {
+    checkArgument(name.matches("^[a-z_]+$"), "name must match [a-z_]");
+    this.name = name;
+    this.keyType = keyType;
+    this.formatter = initFormatter(keyType);
+    this.description = description;
+  }
+
+  /** Name of this field within the metric. */
+  public String getName() {
+    return name;
+  }
+
+  /** Type of value used within the field. */
+  public Class<T> getType() {
+    return keyType;
+  }
+
+  /** Description text for the field explaining its range of values. */
+  public String getDescription() {
+    return description;
+  }
+
+  public Function<T, String> formatter() {
+    return formatter;
+  }
+
+  @SuppressWarnings("unchecked")
+  private static <T> Function<T, String> initFormatter(Class<T> keyType) {
+    if (keyType == String.class) {
+      return (Function<T, String>) Functions.<String> identity();
+
+    } else if (keyType == Integer.class || keyType == Boolean.class) {
+      return (Function<T, String>) Functions.toStringFunction();
+
+    } else if (Enum.class.isAssignableFrom(keyType)) {
+      return new Function<T, String>() {
+        @Override
+        public String apply(T in) {
+          return ((Enum<?>) in).name();
+        }
+      };
+    }
+    throw new IllegalStateException("unsupported type " + keyType.getName());
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/MetricMaker.java

@@ -23,10 +23,28 @@ import java.util.Set;
 /** Factory to create metrics for monitoring. */
 public abstract class MetricMaker {
   /** Metric whose value increments during the life of the process. */
-  public abstract Counter newCounter(String name, Description desc);
+  public abstract Counter0 newCounter(String name, Description desc);
+  public abstract <F1> Counter1<F1> newCounter(
+      String name, Description desc,
+      Field<F1> field1);
+  public abstract <F1, F2> Counter2<F1, F2> newCounter(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2);
+  public abstract <F1, F2, F3> Counter3<F1, F2, F3> newCounter(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2, Field<F3> field3);
 
   /** Metric recording time spent on an operation. */
-  public abstract Timer newTimer(String name, Description desc);
+  public abstract Timer0 newTimer(String name, Description desc);
+  public abstract <F1> Timer1<F1> newTimer(
+      String name, Description desc,
+      Field<F1> field1);
+  public abstract <F1, F2> Timer2<F1, F2> newTimer(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2);
+  public abstract <F1, F2, F3> Timer3<F1, F2, F3> newTimer(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2, Field<F3> field3);
 
   /**
    * Instantaneous reading of a value.
@@ -50,7 +68,7 @@ public abstract class MetricMaker {
    */
   public <V> void newCallbackMetric(String name,
       Class<V> valueClass, Description desc, final Supplier<V> trigger) {
-    final CallbackMetric<V> metric = newCallbackMetric(name, valueClass, desc);
+    final CallbackMetric0<V> metric = newCallbackMetric(name, valueClass, desc);
     newTrigger(metric, new Runnable() {
       @Override
       public void run() {
@@ -59,9 +77,9 @@ public abstract class MetricMaker {
     });
   }
 
-  /** Instantaneous reading of a particular value. */
-  public abstract <V> CallbackMetric<V> newCallbackMetric(String name,
-      Class<V> valueClass, Description desc);
+  /** Instantaneous reading of a single value. */
+  public abstract <V> CallbackMetric0<V> newCallbackMetric(
+      String name, Class<V> valueClass, Description desc);
 
   /** Connect logic to populate a previously created {@link CallbackMetric}. */
   public RegistrationHandle newTrigger(CallbackMetric<?> metric1, Runnable trigger) {

gerrit-server/src/main/java/com/google/gerrit/metrics/Timer0.java

@@ -30,7 +30,7 @@ import java.util.concurrent.TimeUnit;
  * }
  * </pre>
  */
-public abstract class Timer implements RegistrationHandle {
+public abstract class Timer0 implements RegistrationHandle {
   public class Context implements AutoCloseable {
     private final long startNanos;
 

gerrit-server/src/main/java/com/google/gerrit/metrics/Timer1.java

@@ -0,0 +1,61 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import static java.util.concurrent.TimeUnit.NANOSECONDS;
+
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Records elapsed time for an operation or span.
+ * <p>
+ * Typical usage in a try-with-resources block:
+ *
+ * <pre>
+ * try (Timer1.Context ctx = timer.start(field)) {
+ * }
+ * </pre>
+ *
+ * @param <F1> type of the field.
+ */
+public abstract class Timer1<F1> implements RegistrationHandle {
+  public static class Context implements AutoCloseable {
+    private final Timer1<Object> timer;
+    private final Object field1;
+    private final long startNanos;
+
+    @SuppressWarnings("unchecked")
+    <F1> Context(Timer1<F1> timer, F1 field1) {
+      this.timer = (Timer1<Object>) timer;
+      this.field1 = field1;
+      this.startNanos = System.nanoTime();
+    }
+
+    @Override
+    public void close() {
+      timer.record(field1, System.nanoTime() - startNanos, NANOSECONDS);
+    }
+  }
+
+  /** Begin a timer for the current block, value will be recorded when closed. */
+  public Context start(F1 field1) {
+    return new Context(this, field1);
+  }
+
+  /** Record a value in the distribution. */
+  public abstract void record(F1 field1, long value, TimeUnit unit);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/Timer2.java

@@ -0,0 +1,64 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import static java.util.concurrent.TimeUnit.NANOSECONDS;
+
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Records elapsed time for an operation or span.
+ * <p>
+ * Typical usage in a try-with-resources block:
+ *
+ * <pre>
+ * try (Timer2.Context ctx = timer.start(field)) {
+ * }
+ * </pre>
+ *
+ * @param <F1> type of the field.
+ * @param <F2> type of the field.
+ */
+public abstract class Timer2<F1, F2> implements RegistrationHandle {
+  public static class Context implements AutoCloseable {
+    private final Timer2<Object, Object> timer;
+    private final Object field1;
+    private final Object field2;
+    private final long startNanos;
+
+    @SuppressWarnings("unchecked")
+    <F1, F2> Context(Timer2<F1, F2> timer, F1 field1, F2 field2) {
+      this.timer = (Timer2<Object, Object>) timer;
+      this.field1 = field1;
+      this.field2 = field2;
+      this.startNanos = System.nanoTime();
+    }
+
+    @Override
+    public void close() {
+      timer.record(field1, field2, System.nanoTime() - startNanos, NANOSECONDS);
+    }
+  }
+
+  /** Begin a timer for the current block, value will be recorded when closed. */
+  public Context start(F1 field1, F2 field2) {
+    return new Context(this, field1, field2);
+  }
+
+  /** Record a value in the distribution. */
+  public abstract void record(F1 field1, F2 field2, long value, TimeUnit unit);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/Timer3.java

@@ -0,0 +1,69 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import static java.util.concurrent.TimeUnit.NANOSECONDS;
+
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Records elapsed time for an operation or span.
+ * <p>
+ * Typical usage in a try-with-resources block:
+ *
+ * <pre>
+ * try (Timer3.Context ctx = timer.start(field)) {
+ * }
+ * </pre>
+ *
+ * @param <F1> type of the field.
+ * @param <F2> type of the field.
+ * @param <F3> type of the field.
+ */
+public abstract class Timer3<F1, F2, F3> implements RegistrationHandle {
+  public static class Context implements AutoCloseable {
+    private final Timer3<Object, Object, Object> timer;
+    private final Object field1;
+    private final Object field2;
+    private final Object field3;
+    private final long startNanos;
+
+    @SuppressWarnings("unchecked")
+    <F1, F2, F3> Context(Timer3<F1, F2, F3> timer, F1 f1, F2 f2, F3 f3) {
+      this.timer = (Timer3<Object, Object, Object>) timer;
+      this.field1 = f1;
+      this.field2 = f2;
+      this.field3 = f3;
+      this.startNanos = System.nanoTime();
+    }
+
+    @Override
+    public void close() {
+      timer.record(field1, field2, field3,
+          System.nanoTime() - startNanos, NANOSECONDS);
+    }
+  }
+
+  /** Begin a timer for the current block, value will be recorded when closed. */
+  public Context start(F1 field1, F2 field2, F3 field3) {
+    return new Context(this, field1, field2, field3);
+  }
+
+  /** Record a value in the distribution. */
+  public abstract void record(F1 field1, F2 field2, F3 field3,
+      long value, TimeUnit unit);
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedCounter.java

@@ -0,0 +1,109 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.common.base.Function;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Maps;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
+import com.google.gerrit.metrics.dropwizard.DropWizardMetricMaker.CounterImpl;
+
+import com.codahale.metrics.Metric;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+/** Abstract counter broken down into buckets by {@link Field} values. */
+abstract class BucketedCounter implements BucketedMetric {
+  private final DropWizardMetricMaker metrics;
+  private final String name;
+  private final boolean isRate;
+  private final Description.FieldOrdering ordering;
+  protected final Field<?>[] fields;
+  protected final CounterImpl total;
+  private final Map<Object, CounterImpl> cells;
+
+  BucketedCounter(DropWizardMetricMaker metrics,
+      String name, Description desc, Field<?>... fields) {
+    this.metrics = metrics;
+    this.name = name;
+    this.isRate = desc.isRate();
+    this.ordering = desc.getFieldOrdering();
+    this.fields = fields;
+    this.total = metrics.newCounterImpl(name + "_total", isRate);
+    this.cells = new ConcurrentHashMap<>();
+  }
+
+  void doRemove() {
+    for (CounterImpl c : cells.values()) {
+      c.remove();
+    }
+    total.remove();
+    metrics.remove(name);
+  }
+
+  CounterImpl forceCreate(Object f1, Object f2) {
+    return forceCreate(ImmutableList.of(f1, f2));
+  }
+
+  CounterImpl forceCreate(Object f1, Object f2, Object f3) {
+    return forceCreate(ImmutableList.of(f1, f2, f3));
+  }
+
+  CounterImpl forceCreate(Object key) {
+    CounterImpl c = cells.get(key);
+    if (c != null) {
+      return c;
+    }
+
+    synchronized (cells) {
+      c = cells.get(key);
+      if (c == null) {
+        c = metrics.newCounterImpl(submetric(key), isRate);
+        cells.put(key, c);
+      }
+      return c;
+    }
+  }
+
+  private String submetric(Object key) {
+    return DropWizardMetricMaker.name(ordering, name, name(key));
+  }
+
+  abstract String name(Object key);
+
+  @Override
+  public Metric getTotal() {
+    return total.metric;
+  }
+
+  @Override
+  public Field<?>[] getFields() {
+    return fields;
+  }
+
+  @Override
+  public Map<Object, Metric> getCells() {
+    return Maps.transformValues(
+        cells,
+        new Function<CounterImpl, Metric> () {
+          @Override
+          public Metric apply(CounterImpl in) {
+            return in.metric;
+          }
+        });
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedMetric.java

@@ -0,0 +1,29 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.metrics.Field;
+
+import com.codahale.metrics.Metric;
+
+import java.util.Map;
+
+/** Metric broken down into buckets by {@link Field} values. */
+interface BucketedMetric extends Metric {
+  @Nullable Metric getTotal();
+  Field<?>[] getFields();
+  Map<?, Metric> getCells();
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedTimer.java

@@ -0,0 +1,107 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.common.base.Function;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Maps;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
+import com.google.gerrit.metrics.dropwizard.DropWizardMetricMaker.TimerImpl;
+
+import com.codahale.metrics.Metric;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+/** Abstract timer broken down into buckets by {@link Field} values. */
+abstract class BucketedTimer implements BucketedMetric {
+  private final DropWizardMetricMaker metrics;
+  private final String name;
+  private final Description.FieldOrdering ordering;
+  protected final Field<?>[] fields;
+  protected final TimerImpl total;
+  private final Map<Object, TimerImpl> cells;
+
+  BucketedTimer(DropWizardMetricMaker metrics, String name,
+      Description desc, Field<?>... fields) {
+    this.metrics = metrics;
+    this.name = name;
+    this.ordering = desc.getFieldOrdering();
+    this.fields = fields;
+    this.total = metrics.newTimerImpl(name + "_total");
+    this.cells = new ConcurrentHashMap<>();
+  }
+
+  void doRemove() {
+    for (TimerImpl c : cells.values()) {
+      c.remove();
+    }
+    total.remove();
+    metrics.remove(name);
+  }
+
+  TimerImpl forceCreate(Object f1, Object f2) {
+    return forceCreate(ImmutableList.of(f1, f2));
+  }
+
+  TimerImpl forceCreate(Object f1, Object f2, Object f3) {
+    return forceCreate(ImmutableList.of(f1, f2, f3));
+  }
+
+  TimerImpl forceCreate(Object key) {
+    TimerImpl c = cells.get(key);
+    if (c != null) {
+      return c;
+    }
+
+    synchronized (cells) {
+      c = cells.get(key);
+      if (c == null) {
+        c = metrics.newTimerImpl(submetric(key));
+        cells.put(key, c);
+      }
+      return c;
+    }
+  }
+
+  private String submetric(Object key) {
+    return DropWizardMetricMaker.name(ordering, name, name(key));
+  }
+
+  abstract String name(Object key);
+
+  @Override
+  public Metric getTotal() {
+    return total.metric;
+  }
+
+  @Override
+  public Field<?>[] getFields() {
+    return fields;
+  }
+
+  @Override
+  public Map<Object, Metric> getCells() {
+    return Maps.transformValues(
+        cells,
+        new Function<TimerImpl, Metric> () {
+          @Override
+          public Metric apply(TimerImpl in) {
+            return in.metric;
+          }
+        });
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/CallbackGroup.java

@@ -0,0 +1,49 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
+
+class CallbackGroup implements Runnable {
+  private static final long PERIOD = TimeUnit.SECONDS.toNanos(2);
+
+  private final AtomicLong reloadAt;
+  private final Runnable trigger;
+
+  CallbackGroup(Runnable trigger) {
+    this.reloadAt = new AtomicLong(0);
+    this.trigger = trigger;
+  }
+
+  @Override
+  public void run() {
+    if (reload()) {
+      trigger.run();
+    }
+  }
+
+  private boolean reload() {
+    for (;;) {
+      long now = System.nanoTime();
+      long next = reloadAt.get();
+      if (next > now) {
+        return false;
+      } else if (reloadAt.compareAndSet(next, now + PERIOD)) {
+        return true;
+      }
+    }
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/CallbackMetricImpl0.java

@@ -0,0 +1,67 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.gerrit.metrics.CallbackMetric0;
+
+class CallbackMetricImpl0<V> extends CallbackMetric0<V> {
+  @SuppressWarnings("unchecked")
+  static <V> V zeroFor(Class<V> valueClass) {
+    if (valueClass == Integer.class) {
+      return (V) Integer.valueOf(0);
+    } else if (valueClass == Long.class) {
+      return (V) Long.valueOf(0);
+    } else if (valueClass == Double.class) {
+      return (V) Double.valueOf(0);
+    } else if (valueClass == Float.class) {
+      return (V) Float.valueOf(0);
+    } else if (valueClass == String.class) {
+      return (V) "";
+    } else if (valueClass == Boolean.class) {
+      return (V) Boolean.FALSE;
+    } else {
+      throw new IllegalArgumentException("unsupported value type "
+          + valueClass.getName());
+    }
+  }
+
+  final String name;
+  private V value;
+
+  CallbackMetricImpl0(String name, Class<V> valueType) {
+    this.name = name;
+    this.value = zeroFor(valueType);
+  }
+
+  @Override
+  public void set(V value) {
+    this.value = value;
+  }
+
+  @Override
+  public void remove() {
+    // Triggers register and remove the metric.
+  }
+
+  com.codahale.metrics.Gauge<V> gauge(final Runnable trigger) {
+    return new com.codahale.metrics.Gauge<V>() {
+      @Override
+      public V getValue() {
+        trigger.run();
+        return value;
+      }
+    };
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/CounterImpl1.java

@@ -0,0 +1,52 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.common.base.Function;
+import com.google.gerrit.metrics.Counter1;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
+
+/** Optimized version of {@link BucketedCounter} for single dimension. */
+class CounterImpl1<F1> extends BucketedCounter {
+  CounterImpl1(DropWizardMetricMaker metrics, String name, Description desc,
+      Field<F1> field1) {
+    super(metrics, name, desc, field1);
+  }
+
+  Counter1<F1> counter() {
+    return new Counter1<F1>() {
+      @Override
+      public void incrementBy(F1 field1, long value) {
+        total.incrementBy(value);
+        forceCreate(field1).incrementBy(value);
+      }
+
+      @Override
+      public void remove() {
+        doRemove();
+      }
+    };
+  }
+
+  @Override
+  String name(Object field1) {
+    @SuppressWarnings("unchecked")
+    Function<Object, String> fmt =
+        (Function<Object, String>) fields[0].formatter();
+
+    return fmt.apply(field1).replace('/', '-');
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/CounterImplN.java

@@ -0,0 +1,75 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.common.base.Function;
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableList;
+import com.google.gerrit.metrics.Counter2;
+import com.google.gerrit.metrics.Counter3;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
+
+/** Generalized implementation of N-dimensional counter metrics. */
+class CounterImplN extends BucketedCounter implements BucketedMetric {
+  CounterImplN(DropWizardMetricMaker metrics, String name, Description desc,
+      Field<?>... fields) {
+    super(metrics, name, desc, fields);
+  }
+
+  <F1, F2> Counter2<F1, F2> counter2() {
+    return new Counter2<F1, F2>() {
+      @Override
+      public void incrementBy(F1 field1, F2 field2, long value) {
+        total.incrementBy(value);
+        forceCreate(field1, field2).incrementBy(value);
+      }
+
+      @Override
+      public void remove() {
+        doRemove();
+      }
+    };
+  }
+
+  <F1, F2, F3> Counter3<F1, F2, F3> counter3() {
+    return new Counter3<F1, F2, F3>() {
+      @Override
+      public void incrementBy(F1 field1, F2 field2, F3 field3, long value) {
+        total.incrementBy(value);
+        forceCreate(field1, field2, field3).incrementBy(value);
+      }
+
+      @Override
+      public void remove() {
+        doRemove();
+      }
+    };
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  String name(Object key) {
+    ImmutableList<Object> keyList = (ImmutableList<Object>) key;
+    String[] parts = new String[fields.length];
+    for (int i = 0; i < fields.length; i++) {
+      Function<Object, String> fmt =
+          (Function<Object, String>) fields[i].formatter();
+
+      parts[i] = fmt.apply(keyList.get(i)).replace('/', '-');
+    }
+    return Joiner.on('/').join(parts);
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/DropWizardMetricMaker.java

@@ -23,10 +23,19 @@ import com.google.gerrit.extensions.registration.DynamicMap;
 import com.google.gerrit.extensions.registration.RegistrationHandle;
 import com.google.gerrit.extensions.restapi.RestApiModule;
 import com.google.gerrit.metrics.CallbackMetric;
-import com.google.gerrit.metrics.Counter;
+import com.google.gerrit.metrics.CallbackMetric0;
+import com.google.gerrit.metrics.Counter0;
+import com.google.gerrit.metrics.Counter1;
+import com.google.gerrit.metrics.Counter2;
+import com.google.gerrit.metrics.Counter3;
 import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
 import com.google.gerrit.metrics.MetricMaker;
-import com.google.gerrit.metrics.Timer;
+import com.google.gerrit.metrics.Timer0;
+import com.google.gerrit.metrics.Timer1;
+import com.google.gerrit.metrics.Timer2;
+import com.google.gerrit.metrics.Timer3;
+import com.google.gerrit.metrics.Description.FieldOrdering;
 import com.google.inject.Inject;
 import com.google.inject.Scopes;
 import com.google.inject.Singleton;
@@ -35,10 +44,10 @@ import com.codahale.metrics.Metric;
 import com.codahale.metrics.MetricRegistry;
 
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -62,102 +71,174 @@ public class DropWizardMetricMaker extends MetricMaker {
   }
 
   private final MetricRegistry registry;
+  private final Map<String, BucketedMetric> bucketed;
   private final Map<String, ImmutableMap<String, String>> descriptions;
 
   @Inject
   DropWizardMetricMaker(MetricRegistry registry) {
     this.registry = registry;
-    this.descriptions = new HashMap<>();
+    this.bucketed = new ConcurrentHashMap<>();
+    this.descriptions = new ConcurrentHashMap<>();
   }
 
-  Map<String, Metric> getMetricMap() {
-    return registry.getMetrics();
+  Iterable<String> getMetricNames() {
+    return descriptions.keySet();
   }
 
   /** Get the underlying metric implementation. */
   public Metric getMetric(String name) {
-    return registry.getMetrics().get(name);
+    Metric m = bucketed.get(name);
+    return m != null ? m : registry.getMetrics().get(name);
   }
 
-  /** Lookup annotations from a metric's {@link Description}.  */
+  /** Lookup annotations from a metric's {@link Description}. */
   public ImmutableMap<String, String> getAnnotations(String name) {
     return descriptions.get(name);
   }
 
   @Override
-  public synchronized Counter newCounter(String name, Description desc) {
-    checkArgument(!desc.isGauge(), "counters must not be gauge");
-    checkNotDefined(name);
-    descriptions.put(name, desc.getAnnotations());
+  public synchronized Counter0 newCounter(String name, Description desc) {
+    checkCounterDescription(desc);
+    define(name, desc);
+    return newCounterImpl(name, desc.isRate());
+  }
 
-    if (desc.isRate()) {
-      final com.codahale.metrics.Meter metric = registry.meter(name);
-      return new CounterImpl(name) {
+  @Override
+  public synchronized <F1> Counter1<F1> newCounter(
+      String name, Description desc,
+      Field<F1> field1) {
+    checkCounterDescription(desc);
+    CounterImpl1<F1> m = new CounterImpl1<>(this, name, desc, field1);
+    define(name, desc);
+    bucketed.put(name, m);
+    return m.counter();
+  }
+
+  @Override
+  public synchronized <F1, F2> Counter2<F1, F2> newCounter(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2) {
+    checkCounterDescription(desc);
+    CounterImplN m = new CounterImplN(this, name, desc, field1, field2);
+    define(name, desc);
+    bucketed.put(name, m);
+    return m.counter2();
+  }
+
+  @Override
+  public synchronized <F1, F2, F3> Counter3<F1, F2, F3> newCounter(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2, Field<F3> field3) {
+    checkCounterDescription(desc);
+    CounterImplN m = new CounterImplN(this, name, desc, field1, field2, field3);
+    define(name, desc);
+    bucketed.put(name, m);
+    return m.counter3();
+  }
+
+  private static void checkCounterDescription(Description desc) {
+    checkArgument(!desc.isGauge(), "counters must not be gauge");
+  }
+
+  CounterImpl newCounterImpl(String name, boolean isRate) {
+    if (isRate) {
+      final com.codahale.metrics.Meter m = registry.meter(name);
+      return new CounterImpl(name, m) {
         @Override
         public void incrementBy(long delta) {
           checkArgument(delta >= 0, "counter delta must be >= 0");
-          metric.mark(delta);
+          m.mark(delta);
         }
       };
     } else {
-      final com.codahale.metrics.Counter metric = registry.counter(name);
-      return new CounterImpl(name) {
+      final com.codahale.metrics.Counter m = registry.counter(name);
+      return new CounterImpl(name, m) {
         @Override
         public void incrementBy(long delta) {
           checkArgument(delta >= 0, "counter delta must be >= 0");
-          metric.inc(delta);
+          m.inc(delta);
         }
       };
     }
   }
 
   @Override
-  public synchronized Timer newTimer(final String name, Description desc) {
+  public synchronized Timer0 newTimer(String name, Description desc) {
+    checkTimerDescription(desc);
+    define(name, desc);
+    return newTimerImpl(name);
+  }
+
+  @Override
+  public synchronized <F1> Timer1<F1> newTimer(String name, Description desc, Field<F1> field1) {
+    checkTimerDescription(desc);
+    TimerImpl1<F1> m = new TimerImpl1<>(this, name, desc, field1);
+    define(name, desc);
+    bucketed.put(name, m);
+    return m.timer();
+  }
+
+  @Override
+  public synchronized <F1, F2> Timer2<F1, F2> newTimer(String name, Description desc,
+      Field<F1> field1, Field<F2> field2) {
+    checkTimerDescription(desc);
+    TimerImplN m = new TimerImplN(this, name, desc, field1, field2);
+    define(name, desc);
+    bucketed.put(name, m);
+    return m.timer2();
+  }
+
+  @Override
+  public synchronized <F1, F2, F3> Timer3<F1, F2, F3> newTimer(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2, Field<F3> field3) {
+    checkTimerDescription(desc);
+    TimerImplN m = new TimerImplN(this, name, desc, field1, field2, field3);
+    define(name, desc);
+    bucketed.put(name, m);
+    return m.timer3();
+  }
+
+  private static void checkTimerDescription(Description desc) {
     checkArgument(!desc.isGauge(), "timer must not be a gauge");
     checkArgument(!desc.isRate(), "timer must not be a rate");
     checkArgument(desc.isCumulative(), "timer must be cumulative");
     checkArgument(desc.getTimeUnit() != null, "timer must have a unit");
-    checkNotDefined(name);
-    descriptions.put(name, desc.getAnnotations());
-
-    final com.codahale.metrics.Timer metric = registry.timer(name);
-    return new Timer() {
-      @Override
-      public void record(long value, TimeUnit unit) {
-        checkArgument(value >= 0, "timer delta must be >= 0");
-        metric.update(value, unit);
-      }
-
-      @Override
-      public void remove() {
-        descriptions.remove(name);
-        registry.remove(name);
-      }
-    };
   }
 
-  @SuppressWarnings("unused")
+  TimerImpl newTimerImpl(String name) {
+    return new TimerImpl(name, registry.timer(name));
+  }
+
   @Override
-  public <V> CallbackMetric<V> newCallbackMetric(String name,
-      Class<V> valueClass, Description desc) {
-    checkNotDefined(name);
-    descriptions.put(name, desc.getAnnotations());
-    return new CallbackMetricImpl<V>(name, valueClass);
+  public <V> CallbackMetric0<V> newCallbackMetric(
+      String name, Class<V> valueClass, Description desc) {
+    define(name, desc);
+    return new CallbackMetricImpl0<>(name, valueClass);
   }
 
   @Override
   public synchronized RegistrationHandle newTrigger(
       Set<CallbackMetric<?>> metrics, Runnable trigger) {
+    if (metrics.size() > 1) {
+      trigger = new CallbackGroup(trigger);
+    }
+
     for (CallbackMetric<?> m : metrics) {
-      checkNotDefined(((CallbackMetricImpl<?>) m).name);
+      CallbackMetricImpl0<?> metric = (CallbackMetricImpl0<?>) m;
+      if (registry.getMetrics().containsKey(metric.name)) {
+        throw new IllegalStateException(String.format(
+            "metric %s already configured", metric.name));
+      }
     }
 
     final List<String> names = new ArrayList<>(metrics.size());
     for (CallbackMetric<?> m : metrics) {
-      CallbackMetricImpl<?> metric = (CallbackMetricImpl<?>) m;
+      CallbackMetricImpl0<?> metric = (CallbackMetricImpl0<?>) m;
       registry.register(metric.name, metric.gauge(trigger));
       names.add(metric.name);
     }
+
     return new RegistrationHandle() {
       @Override
       public void remove() {
@@ -169,18 +250,40 @@ public class DropWizardMetricMaker extends MetricMaker {
     };
   }
 
-  private void checkNotDefined(String name) {
-    if (registry.getNames().contains(name)) {
+  synchronized void remove(String name) {
+    bucketed.remove(name);
+    descriptions.remove(name);
+  }
+
+  private synchronized void define(String name, Description desc) {
+    if (descriptions.containsKey(name)) {
       throw new IllegalStateException(String.format(
           "metric %s already defined", name));
     }
+    descriptions.put(name, desc.getAnnotations());
   }
 
-  private abstract class CounterImpl extends Counter {
-    private final String name;
+  static String name(Description.FieldOrdering ordering,
+      String codeName,
+      String fieldValues) {
+    if (ordering == FieldOrdering.PREFIX_FIELDS_BASENAME) {
+      int s = codeName.lastIndexOf('/');
+      if (s > 0) {
+        String prefix = codeName.substring(0, s);
+        String metric = codeName.substring(s + 1);
+        return prefix + '/' + fieldValues + '/' + metric;
+      }
+    }
+    return codeName + '/' + fieldValues;
+  }
 
-    CounterImpl(String name) {
+  abstract class CounterImpl extends Counter0 {
+    private final String name;
+    final Metric metric;
+
+    CounterImpl(String name, Metric metric) {
       this.name = name;
+      this.metric = metric;
     }
 
     @Override
@@ -190,50 +293,25 @@ public class DropWizardMetricMaker extends MetricMaker {
     }
   }
 
-  private static class CallbackMetricImpl<V> extends CallbackMetric<V> {
+  class TimerImpl extends Timer0 {
     private final String name;
-    private V value;
+    final com.codahale.metrics.Timer metric;
 
-    @SuppressWarnings("unchecked")
-    CallbackMetricImpl(String name, Class<V> valueClass) {
+    private TimerImpl(String name, com.codahale.metrics.Timer metric) {
       this.name = name;
-
-      if (valueClass == Integer.class) {
-        value = (V) Integer.valueOf(0);
-      } else if (valueClass == Long.class) {
-        value = (V) Long.valueOf(0);
-      } else if (valueClass == Double.class) {
-        value = (V) Double.valueOf(0);
-      } else if (valueClass == Float.class) {
-        value = (V) Float.valueOf(0);
-      } else if (valueClass == String.class) {
-        value = (V) "";
-      } else if (valueClass == Boolean.class) {
-        value = (V) Boolean.FALSE;
-      } else {
-        throw new IllegalArgumentException("unsupported value type "
-            + valueClass.getName());
-      }
+      this.metric = metric;
     }
 
     @Override
-    public void set(V value) {
-      this.value = value;
+    public void record(long value, TimeUnit unit) {
+      checkArgument(value >= 0, "timer delta must be >= 0");
+      metric.update(value, unit);
     }
 
     @Override
     public void remove() {
-      // Triggers register and remove the metric.
-    }
-
-    com.codahale.metrics.Gauge<V> gauge(final Runnable trigger) {
-      return new com.codahale.metrics.Gauge<V>() {
-        @Override
-        public V getValue() {
-          trigger.run();
-          return value;
-        }
-      };
+      descriptions.remove(name);
+      registry.remove(name);
     }
   }
 }

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/ListMetrics.java

@@ -55,14 +55,12 @@ class ListMetrics implements RestReadView<ConfigResource> {
     }
 
     SortedMap<String, MetricJson> out = new TreeMap<>();
-    Map<String, Metric> all = metrics.getMetricMap();
-
     List<String> prefixes = new ArrayList<>(query.size());
     for (String q : query) {
       if (q.endsWith("/")) {
         prefixes.add(q);
       } else {
-        Metric m = all.get(q);
+        Metric m = metrics.getMetric(q);
         if (m != null) {
           out.put(q, toJson(q, m));
         }
@@ -70,10 +68,9 @@ class ListMetrics implements RestReadView<ConfigResource> {
     }
 
     if (query.isEmpty() || !prefixes.isEmpty()) {
-      for (Map.Entry<String, Metric> e : all.entrySet()) {
-        String name = e.getKey();
+      for (String name : metrics.getMetricNames()) {
         if (include(prefixes, name)) {
-          out.put(name, toJson(name, e.getValue()));
+          out.put(name, toJson(name, metrics.getMetric(name)));
         }
       }
     }

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/MetricJson.java

@@ -14,8 +14,11 @@
 
 package com.google.gerrit.metrics.dropwizard;
 
+import com.google.common.base.Function;
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
 
 import com.codahale.metrics.Counter;
 import com.codahale.metrics.Gauge;
@@ -24,6 +27,11 @@ import com.codahale.metrics.Metric;
 import com.codahale.metrics.Snapshot;
 import com.codahale.metrics.Timer;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+
 class MetricJson {
   String description;
   String unit;
@@ -50,6 +58,9 @@ class MetricJson {
   Double max;
   Double std_dev;
 
+  List<FieldJson> fields;
+  Map<String, Object> buckets;
+
   MetricJson(Metric metric, ImmutableMap<String, String> atts, boolean dataOnly) {
     if (!dataOnly) {
       description = atts.get(Description.DESCRIPTION);
@@ -62,7 +73,20 @@ class MetricJson {
   }
 
   private void init(Metric metric, ImmutableMap<String, String> atts) {
-    if (metric instanceof Counter) {
+    if (metric instanceof BucketedMetric) {
+      BucketedMetric m = (BucketedMetric) metric;
+      if (m.getTotal() != null) {
+        init(m.getTotal(), atts);
+      }
+
+      Field<?>[] fieldList = m.getFields();
+      fields = new ArrayList<>(fieldList.length);
+      for (Field<?> f : fieldList) {
+        fields.add(new FieldJson(f));
+      }
+      buckets = makeBuckets(fieldList, m.getCells(), atts);
+
+    } else if (metric instanceof Counter) {
       Counter c = (Counter) metric;
       count = c.getCount();
 
@@ -103,4 +127,61 @@ class MetricJson {
   private static Boolean toBool(ImmutableMap<String, String> atts, String key) {
     return Description.TRUE_VALUE.equals(atts.get(key)) ? true : null;
   }
+
+  @SuppressWarnings("unchecked")
+  private static Map<String, Object> makeBuckets(
+      Field<?>[] fields,
+      Map<?, Metric> metrics,
+      ImmutableMap<String, String> atts) {
+    if (fields.length == 1) {
+      Function<Object, String> fmt =
+          (Function<Object, String>) fields[0].formatter();
+      Map<String, Object> out = new TreeMap<>();
+      for (Map.Entry<?, Metric> e : metrics.entrySet()) {
+        out.put(
+            fmt.apply(e.getKey()),
+            new MetricJson(e.getValue(), atts, true));
+      }
+      return out;
+    }
+
+    Map<String, Object> out = new TreeMap<>();
+    for (Map.Entry<?, Metric> e : metrics.entrySet()) {
+      ImmutableList<Object> keys = (ImmutableList<Object>) e.getKey();
+      Map<String, Object> dst = out;
+
+      for (int i = 0; i < fields.length - 1; i++) {
+        Function<Object, String> fmt =
+            (Function<Object, String>) fields[i].formatter();
+        String key = fmt.apply(keys.get(i));
+        Map<String, Object> t = (Map<String, Object>) dst.get(key);
+        if (t == null) {
+          t = new TreeMap<>();
+          dst.put(key, t);
+        }
+        dst = t;
+      }
+
+      Function<Object, String> fmt =
+          (Function<Object, String>) fields[fields.length - 1].formatter();
+      dst.put(
+          fmt.apply(keys.get(fields.length - 1)),
+          new MetricJson(e.getValue(), atts, true));
+    }
+    return out;
+  }
+
+  static class FieldJson {
+    String name;
+    String type;
+    String description;
+
+    FieldJson(Field<?> field) {
+      this.name = field.getName();
+      this.description = field.getDescription();
+      this.type = Enum.class.isAssignableFrom(field.getType())
+          ? field.getType().getSimpleName()
+          : null;
+    }
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/TimerImpl1.java

@@ -0,0 +1,54 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.common.base.Function;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
+import com.google.gerrit.metrics.Timer1;
+
+import java.util.concurrent.TimeUnit;
+
+/** Optimized version of {@link BucketedTimer} for single dimension. */
+class TimerImpl1<F1> extends BucketedTimer implements BucketedMetric {
+  TimerImpl1(DropWizardMetricMaker metrics, String name,
+      Description desc, Field<F1> field1) {
+    super(metrics, name, desc, field1);
+  }
+
+  Timer1<F1> timer() {
+    return new Timer1<F1>() {
+      @Override
+      public void record(F1 field1, long value, TimeUnit unit) {
+        total.record(value, unit);
+        forceCreate(field1).record(value, unit);
+      }
+
+      @Override
+      public void remove() {
+        doRemove();
+      }
+    };
+  }
+
+  @Override
+  String name(Object field1) {
+    @SuppressWarnings("unchecked")
+    Function<Object, String> fmt =
+        (Function<Object, String>) fields[0].formatter();
+
+    return fmt.apply(field1).replace('/', '-');
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/TimerImplN.java

@@ -0,0 +1,78 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.google.common.base.Function;
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableList;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
+import com.google.gerrit.metrics.Timer2;
+import com.google.gerrit.metrics.Timer3;
+
+import java.util.concurrent.TimeUnit;
+
+/** Generalized implementation of N-dimensional timer metrics. */
+class TimerImplN extends BucketedTimer implements BucketedMetric {
+  TimerImplN(DropWizardMetricMaker metrics, String name,
+      Description desc, Field<?>... fields) {
+    super(metrics, name, desc, fields);
+  }
+
+  <F1, F2> Timer2<F1, F2> timer2() {
+    return new Timer2<F1, F2>() {
+      @Override
+      public void record(F1 field1, F2 field2, long value, TimeUnit unit) {
+        total.record(value, unit);
+        forceCreate(field1, field2).record(value, unit);
+      }
+
+      @Override
+      public void remove() {
+        doRemove();
+      }
+    };
+  }
+
+  <F1, F2, F3> Timer3<F1, F2, F3> timer3() {
+    return new Timer3<F1, F2, F3>() {
+      @Override
+      public void record(F1 field1, F2 field2, F3 field3,
+          long value, TimeUnit unit) {
+        total.record(value, unit);
+        forceCreate(field1, field2, field3).record(value, unit);
+      }
+
+      @Override
+      public void remove() {
+        doRemove();
+      }
+    };
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  String name(Object key) {
+    ImmutableList<Object> keyList = (ImmutableList<Object>) key;
+    String[] parts = new String[fields.length];
+    for (int i = 0; i < fields.length; i++) {
+      Function<Object, String> fmt =
+          (Function<Object, String>) fields[i].formatter();
+
+      parts[i] = fmt.apply(keyList.get(i)).replace('/', '-');
+    }
+    return Joiner.on('/').join(parts);
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/git/UploadPackMetricsHook.java

@@ -14,8 +14,9 @@
 
 package com.google.gerrit.server.git;
 
-import com.google.gerrit.metrics.Counter;
+import com.google.gerrit.metrics.Counter1;
 import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
 import com.google.gerrit.metrics.MetricMaker;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
@@ -25,7 +26,12 @@ import org.eclipse.jgit.transport.PostUploadHook;
 
 @Singleton
 public class UploadPackMetricsHook implements PostUploadHook {
-  private final Counter upload;
+  enum Operation {
+    CLONE,
+    FETCH;
+  }
+
+  private final Counter1<Operation> upload;
 
   @Inject
   UploadPackMetricsHook(MetricMaker metricMaker) {
@@ -33,11 +39,17 @@ public class UploadPackMetricsHook implements PostUploadHook {
         "git/upload-pack",
         new Description("Total number of git-upload-pack requests")
           .setRate()
-          .setUnit("requests"));
+          .setUnit("requests"),
+        Field.ofEnum(Operation.class, "operation"));
   }
 
   @Override
   public void onPostUpload(PackStatistics stats) {
-    upload.increment();
+    Operation op = Operation.FETCH;
+    if (stats.getUninterestingObjects() == null
+        || stats.getUninterestingObjects().isEmpty()) {
+      op = Operation.CLONE;
+    }
+    upload.increment(op);
   }
 }

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginMetricMaker.java

@@ -17,10 +17,18 @@ package com.google.gerrit.server.plugins;
 import com.google.gerrit.extensions.events.LifecycleListener;
 import com.google.gerrit.extensions.registration.RegistrationHandle;
 import com.google.gerrit.metrics.CallbackMetric;
-import com.google.gerrit.metrics.Counter;
+import com.google.gerrit.metrics.CallbackMetric0;
+import com.google.gerrit.metrics.Counter0;
+import com.google.gerrit.metrics.Counter1;
+import com.google.gerrit.metrics.Counter2;
+import com.google.gerrit.metrics.Counter3;
 import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Field;
 import com.google.gerrit.metrics.MetricMaker;
-import com.google.gerrit.metrics.Timer;
+import com.google.gerrit.metrics.Timer0;
+import com.google.gerrit.metrics.Timer1;
+import com.google.gerrit.metrics.Timer2;
+import com.google.gerrit.metrics.Timer3;
 
 import java.util.Collections;
 import java.util.HashSet;
@@ -39,23 +47,79 @@ class PluginMetricMaker extends MetricMaker implements LifecycleListener {
   }
 
   @Override
-  public Counter newCounter(String name, Description desc) {
-    Counter m = root.newCounter(prefix + name, desc);
+  public Counter0 newCounter(String name, Description desc) {
+    Counter0 m = root.newCounter(prefix + name, desc);
     cleanup.add(m);
     return m;
   }
 
   @Override
-  public Timer newTimer(String name, Description desc) {
-    Timer m = root.newTimer(prefix + name, desc);
+  public <F1> Counter1<F1> newCounter(
+      String name, Description desc,
+      Field<F1> field1) {
+    Counter1<F1> m = root.newCounter(prefix + name, desc, field1);
     cleanup.add(m);
     return m;
   }
 
   @Override
-  public <V> CallbackMetric<V> newCallbackMetric(String name,
-      Class<V> valueClass, Description desc) {
-    CallbackMetric<V> m = root.newCallbackMetric(prefix + name, valueClass, desc);
+  public <F1, F2> Counter2<F1, F2> newCounter(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2) {
+    Counter2<F1, F2> m = root.newCounter(prefix + name, desc, field1, field2);
+    cleanup.add(m);
+    return m;
+  }
+
+  @Override
+  public <F1, F2, F3> Counter3<F1, F2, F3> newCounter(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2, Field<F3> field3) {
+    Counter3<F1, F2, F3> m =
+        root.newCounter(prefix + name, desc, field1, field2, field3);
+    cleanup.add(m);
+    return m;
+  }
+
+  @Override
+  public Timer0 newTimer(String name, Description desc) {
+    Timer0 m = root.newTimer(prefix + name, desc);
+    cleanup.add(m);
+    return m;
+  }
+
+  @Override
+  public <F1> Timer1<F1> newTimer(
+      String name, Description desc,
+      Field<F1> field1) {
+    Timer1<F1> m = root.newTimer(prefix + name, desc, field1);
+    cleanup.add(m);
+    return m;
+  }
+
+  @Override
+  public <F1, F2> Timer2<F1, F2> newTimer(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2) {
+    Timer2<F1, F2> m = root.newTimer(prefix + name, desc, field1, field2);
+    cleanup.add(m);
+    return m;
+  }
+
+  @Override
+  public <F1, F2, F3> Timer3<F1, F2, F3> newTimer(
+      String name, Description desc,
+      Field<F1> field1, Field<F2> field2, Field<F3> field3) {
+    Timer3<F1, F2, F3> m =
+        root.newTimer(prefix + name, desc, field1, field2, field3);
+    cleanup.add(m);
+    return m;
+  }
+
+  @Override
+  public <V> CallbackMetric0<V> newCallbackMetric(
+      String name, Class<V> valueClass, Description desc) {
+    CallbackMetric0<V> m = root.newCallbackMetric(prefix + name, valueClass, desc);
     cleanup.add(m);
     return m;
   }

gerrit-server/src/main/java/com/google/gerrit/server/query/change/QueryProcessor.java

@@ -22,7 +22,7 @@ import com.google.common.collect.Ordering;
 import com.google.gerrit.common.data.GlobalCapability;
 import com.google.gerrit.metrics.Description;
 import com.google.gerrit.metrics.MetricMaker;
-import com.google.gerrit.metrics.Timer;
+import com.google.gerrit.metrics.Timer0;
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.server.CurrentUser;
 import com.google.gerrit.server.index.IndexConfig;
@@ -122,7 +122,7 @@ public class QueryProcessor {
       List<Predicate<ChangeData>> queries)
       throws OrmException, QueryParseException {
     @SuppressWarnings("resource")
-    Timer.Context context = metrics.executionTime.start();
+    Timer0.Context context = metrics.executionTime.start();
 
     Predicate<ChangeData> visibleToMe = enforceVisibility
         ? new IsVisibleToPredicate(db, changeControlFactory, userProvider.get())
@@ -217,7 +217,7 @@ public class QueryProcessor {
 
   @Singleton
   static class Metrics {
-    final Timer executionTime;
+    final Timer0 executionTime;
 
     @Inject
     Metrics(MetricMaker metricMaker) {

gerrit-sshd/src/main/java/com/google/gerrit/sshd/SshDaemon.java

@@ -24,7 +24,7 @@ import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
 import com.google.gerrit.common.Version;
 import com.google.gerrit.extensions.events.LifecycleListener;
-import com.google.gerrit.metrics.Counter;
+import com.google.gerrit.metrics.Counter0;
 import com.google.gerrit.metrics.Description;
 import com.google.gerrit.metrics.MetricMaker;
 import com.google.gerrit.server.config.ConfigUtil;
@@ -266,7 +266,7 @@ public class SshDaemon extends SshServer implements SshInfo, LifecycleListener {
           }
         });
 
-    final Counter sesssionsCreated = metricMaker.newCounter(
+    final Counter0 sesssionsCreated = metricMaker.newCounter(
         "sshd/sessions/created",
         new Description("Rate of new SSH sessions")
           .setRate()