Extract metrics package from gerrit-server top-level directory

Change-Id: I7c21b82de289ebc24b530d6b37abf6c33b4dec2f

java/com/google/gerrit/metrics/Counter0.java

@@ -0,0 +1,41 @@
+// 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}.
+ */
+public abstract class Counter0 implements RegistrationHandle {
+  /** Increment the counter by one event. */
+  public void increment() {
+    incrementBy(1);
+  }
+
+  /**
+   * Increment the counter by a specified amount.
+   *
+   * @param value value to increment by, must be &gt;= 0.
+   */
+  public abstract void incrementBy(long value);
+}