Extract metrics package from gerrit-server top-level directory

Change-Id: I7c21b82de289ebc24b530d6b37abf6c33b4dec2f

java/com/google/gerrit/metrics/Histogram2.java

@@ -0,0 +1,36 @@
+// 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;
+
+/**
+ * Measures the statistical distribution of values in a stream of data.
+ *
+ * <p>Suitable uses are "response size in bytes", etc.
+ *
+ * @param <F1> type of the field.
+ * @param <F2> type of the field.
+ */
+public abstract class Histogram2<F1, F2> implements RegistrationHandle {
+  /**
+   * Record a sample of a specified amount.
+   *
+   * @param field1 bucket to record sample
+   * @param field2 bucket to record sample
+   * @param value value to record
+   */
+  public abstract void record(F1 field1, F2 field2, long value);
+}