Add StackDriver implementation, in monitoring/metrics package

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130287319

java/google/registry/monitoring/metrics/IncrementableMetric.java

@@ -0,0 +1,35 @@
+// Copyright 2016 The Domain Registry Authors. All Rights Reserved.
+//
+// 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 google.registry.monitoring.metrics;
+
+/**
+ * A {@link Metric} which can be incremented.
+ *
+ * <p>This is a view into a {@link Counter} to provide compile-time checking to disallow re-setting
+ * the metric, which is useful for metrics which should be monotonic.
+ */
+public interface IncrementableMetric extends Metric<Long> {
+
+  /**
+   * Increments a metric for a given set of label values.
+   *
+   * <p>If the metric is undefined for given label values, it will first be set to zero.
+   *
+   * <p>The metric's timestamp will be updated to the current time for the given label values.
+   *
+   * <p>The count of {@code labelValues} must be equal to the underlying metric's count of labels.
+   */
+  void incrementBy(long offset, String... labelValues);
+}