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.

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

java/google/registry/monitoring/metrics/Distribution.java

@@ -0,0 +1,46 @@
+// 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.ImmutableRangeMap;
+
+/**
+ * Models a distribution of double-precision floating point sample data, and provides summary
+ * statistics of the distribution. This class also models the probability density function (PDF) of
+ * the distribution with a histogram.
+ *
+ * <p>The summary statistics provided are the mean and sumOfSquaredDeviation of the distribution.
+ *
+ * <p>The histogram fitting function is provided via a {@link DistributionFitter} implementation.
+ *
+ * @see DistributionFitter
+ */
+public interface Distribution {
+
+  /** Returns the mean of this distribution. */
+  double mean();
+
+  /** Returns the sum of squared deviations from the mean of this distribution. */
+  double sumOfSquaredDeviation();
+
+  /** Returns the count of samples in this distribution. */
+  long count();
+
+  /** Returns a histogram of the distribution's values. */
+  ImmutableRangeMap<Double, Long> intervalCounts();
+
+  /** Returns the {@link DistributionFitter} of this distribution. */
+  DistributionFitter distributionFitter();
+}