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/MetricWriter.java

@@ -0,0 +1,36 @@
+// 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;
+
+import java.io.Flushable;
+import java.io.IOException;
+
+/** An interface for exporting Metrics. */
+public interface MetricWriter extends Flushable {
+
+  /**
+   * Writes a {@link MetricPoint} to the writer's destination.
+   *
+   * <p>The write may be asynchronous.
+   *
+   * @throws IOException if the provided metric cannot be represented by the writer or if the metric
+   *     cannot be flushed.
+   */
+  <V> void write(MetricPoint<V> metricPoint) throws IOException;
+
+  /** Forces the writer to synchronously write all buffered metric values. */
+  @Override
+  void flush() throws IOException;
+}