DropWizard metric support

Gerrit server supports defining and recording metrics.  Metric
reporters for monitoring can be implemented as plugins.  A basic
Graphite reporter is available here:

  https://gerrit-review.googlesource.com/#/c/72202/

Some example metrics are included in this change:

  change/query/query_latency
  (Query latency)

  sshd/sessions/connected
  (SSH sessions connected)

  sshd/sessions/created/count
  (SSH connections created)

  git/upload-pack
  (Upload packs requests)

Partially-by: Gustaf Lundh <gustaflh@axis.com>
Change-Id: I46a07aace57efe236ee724ec8d34c581e2c37965

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

@@ -0,0 +1,83 @@
+// 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.common.base.Supplier;
+import com.google.common.collect.ImmutableSet;
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+
+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);
+
+  /** Metric recording time spent on an operation. */
+  public abstract Timer newTimer(String name, Description desc);
+
+  /**
+   * Instantaneous reading of a value.
+   *
+   * <pre>
+   * metricMaker.newCallbackMetric(&quot;memory&quot;,
+   *     new Description(&quot;Total bytes of memory used&quot;)
+   *        .setGauge()
+   *        .setUnit(Units.BYTES),
+   *     new Supplier&lt;Long&gt;() {
+   *       public Long get() {
+   *         return Runtime.getRuntime().totalMemory();
+   *       }
+   *     });
+   * </pre>
+   *
+   * @param name unique name of the metric.
+   * @param valueClass type of value recorded by the metric.
+   * @param desc description of the metric.
+   * @param trigger function to compute the value of the metric.
+   */
+  public <V> void newCallbackMetric(String name,
+      Class<V> valueClass, Description desc, final Supplier<V> trigger) {
+    final CallbackMetric<V> metric = newCallbackMetric(name, valueClass, desc);
+    newTrigger(metric, new Runnable() {
+      @Override
+      public void run() {
+        metric.set(trigger.get());
+      }
+    });
+  }
+
+  /** Instantaneous reading of a particular value. */
+  public abstract <V> CallbackMetric<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) {
+    return newTrigger(ImmutableSet.<CallbackMetric<?>>of(metric1), trigger);
+  }
+
+  public RegistrationHandle newTrigger(CallbackMetric<?> metric1,
+      CallbackMetric<?> metric2, Runnable trigger) {
+    return newTrigger(ImmutableSet.of(metric1, metric2), trigger);
+  }
+
+  public RegistrationHandle newTrigger(CallbackMetric<?> metric1,
+      CallbackMetric<?> metric2, CallbackMetric<?> metric3, Runnable trigger) {
+    return newTrigger(ImmutableSet.of(metric1, metric2, metric3), trigger);
+  }
+
+  public abstract RegistrationHandle newTrigger(Set<CallbackMetric<?>> metrics,
+      Runnable trigger);
+}