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/MetricExporter.java

@@ -0,0 +1,52 @@
+// 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 com.google.common.base.Optional;
+import com.google.common.collect.ImmutableList;
+import com.google.common.util.concurrent.AbstractExecutionThreadService;
+import java.util.concurrent.BlockingQueue;
+
+/**
+ * Background service to asynchronously push bundles of {@link MetricPoint} instances to a {@link
+ * MetricWriter}.
+ */
+class MetricExporter extends AbstractExecutionThreadService {
+
+  private final BlockingQueue<Optional<ImmutableList<MetricPoint<?>>>> writeQueue;
+  private final MetricWriter writer;
+
+  MetricExporter(
+      BlockingQueue<Optional<ImmutableList<MetricPoint<?>>>> writeQueue, MetricWriter writer) {
+    this.writeQueue = writeQueue;
+    this.writer = writer;
+  }
+
+  @Override
+  protected void run() throws Exception {
+    while (isRunning()) {
+      Optional<ImmutableList<MetricPoint<?>>> batch = writeQueue.take();
+      if (batch.isPresent()) {
+        for (MetricPoint<?> point : batch.get()) {
+          writer.write(point);
+        }
+        writer.flush();
+      } else {
+        // An absent optional indicates that the Reporter wants this service to shut down.
+        return;
+      }
+    }
+  }
+}