Add the Distribution data type for instrumentation

This is one of a series of CLs adding a new metric type, EventMetric, which
is used for tracking numerical distributions.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132103552

java/google/registry/monitoring/metrics/ImmutableDistribution.java

@@ -0,0 +1,54 @@
+// 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.auto.value.AutoValue;
+import com.google.common.collect.ImmutableRangeMap;
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * An immutable {@link Distribution}. Instances of this class can used to create {@link MetricPoint}
+ * instances, and should be used when exporting values to a {@link MetricWriter}.
+ *
+ * @see MutableDistribution
+ */
+@ThreadSafe
+@AutoValue
+public abstract class ImmutableDistribution implements Distribution {
+
+  public static ImmutableDistribution copyOf(Distribution distribution) {
+    return new AutoValue_ImmutableDistribution(
+        distribution.mean(),
+        distribution.sumOfSquaredDeviation(),
+        distribution.count(),
+        distribution.intervalCounts(),
+        distribution.distributionFitter());
+  }
+
+  @Override
+  public abstract double mean();
+
+  @Override
+  public abstract double sumOfSquaredDeviation();
+
+  @Override
+  public abstract long count();
+
+  @Override
+  public abstract ImmutableRangeMap<Double, Long> intervalCounts();
+
+  @Override
+  public abstract DistributionFitter distributionFitter();
+}