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/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);
+}