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/Metric.java

@@ -0,0 +1,43 @@
+// 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.collect.ImmutableList;
+
+/**
+ * A Metric which stores timestamped values.
+ *
+ * <p>This is a read-only view.
+ */
+public interface Metric<V> {
+
+  /**
+   * Returns the list of the latest {@link MetricPoint} instances for every label-value combination
+   * tracked for this metric.
+   */
+  ImmutableList<MetricPoint<V>> getTimestampedValues();
+
+  /** Returns the count of values being stored with this metric. */
+  int getCardinality();
+
+  /** Returns the schema of this metric. */
+  MetricSchema getMetricSchema();
+
+  /**
+   * Returns the type for the value of this metric, which would otherwise be erased at compile-time.
+   * This is useful for implementors of {@link MetricWriter}.
+   */
+  Class<V> getValueClass();
+}