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/DistributionFitter.java

@@ -0,0 +1,33 @@
+// 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.ImmutableSortedSet;
+
+/**
+ * A companion interface to {@link Distribution} which fits samples to a histogram in order to
+ * estimate the probability density function (PDF) of the {@link Distribution}.
+ *
+ * <p>The fitter models the histogram with a set of finite boundaries. The closed-open interval
+ * [a,b) between two consecutive boundaries represents the domain of permissible values in that
+ * interval. The values less than the first boundary are in the underflow interval of (-inf, a) and
+ * values greater or equal to the last boundary in the array are in the overflow interval of [a,
+ * inf).
+ */
+public interface DistributionFitter {
+
+  /** Returns a sorted set of the boundaries modeled by this {@link DistributionFitter}. */
+  ImmutableSortedSet<Double> boundaries();
+}