Add StackDriver implementation, in monitoring/metrics package

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

java/google/registry/monitoring/metrics/Counter.java

@@ -0,0 +1,97 @@
+// 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 static com.google.common.base.Preconditions.checkArgument;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.util.concurrent.AtomicLongMap;
+import google.registry.monitoring.metrics.MetricSchema.Kind;
+import java.util.Map.Entry;
+import javax.annotation.concurrent.ThreadSafe;
+import org.joda.time.Instant;
+
+/**
+ * A metric which stores Long values. It is stateful and can be changed in increments.
+ *
+ * <p>This metric is generally suitable for counters, such as requests served or errors generated.
+ */
+@ThreadSafe
+public final class Counter extends AbstractMetric<Long>
+    implements SettableMetric<Long>, IncrementableMetric {
+
+  private static final String LABEL_COUNT_ERROR =
+      "The count of labelValues must be equal to the underlying "
+          + "MetricDescriptor's count of labels.";
+
+  private final AtomicLongMap<ImmutableList<String>> values = AtomicLongMap.create();
+
+  Counter(
+      String name,
+      String description,
+      String valueDisplayName,
+      ImmutableSet<LabelDescriptor> labels) {
+    super(name, description, valueDisplayName, Kind.CUMULATIVE, labels, Long.class);
+  }
+
+  @VisibleForTesting
+  void incrementBy(Number offset, ImmutableList<String> labelValues) {
+    values.addAndGet(labelValues, offset.longValue());
+  }
+
+  @Override
+  public final void incrementBy(long offset, String... labelValues) {
+    checkArgument(labelValues.length == this.getMetricSchema().labels().size(), LABEL_COUNT_ERROR);
+
+    incrementBy(offset, ImmutableList.copyOf(labelValues));
+  }
+
+  /**
+   * Returns a snapshot of the metric's values. The timestamp of each {@link MetricPoint} will be
+   * the last modification time for that tuple of label values.
+   */
+  @Override
+  public final ImmutableList<MetricPoint<Long>> getTimestampedValues() {
+    return getTimestampedValues(Instant.now());
+  }
+
+  @Override
+  public final int getCardinality() {
+    return values.size();
+  }
+
+  @VisibleForTesting
+  final ImmutableList<MetricPoint<Long>> getTimestampedValues(Instant timestamp) {
+    ImmutableList.Builder<MetricPoint<Long>> timestampedValues = new ImmutableList.Builder<>();
+    for (Entry<ImmutableList<String>, Long> entry : values.asMap().entrySet()) {
+      timestampedValues.add(MetricPoint.create(this, entry.getKey(), timestamp, entry.getValue()));
+    }
+    return timestampedValues.build();
+  }
+
+  @VisibleForTesting
+  final void set(Long value, ImmutableList<String> labelValues) {
+    this.values.put(labelValues, value);
+  }
+
+  @Override
+  public final void set(Long value, String... labelValues) {
+    checkArgument(labelValues.length == this.getMetricSchema().labels().size(), LABEL_COUNT_ERROR);
+
+    set(value, ImmutableList.copyOf(labelValues));
+  }
+}