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/LinearFitter.java

@@ -0,0 +1,64 @@
+// 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 static google.registry.monitoring.metrics.MetricsUtils.checkDouble;
+
+import com.google.auto.value.AutoValue;
+import com.google.common.collect.ImmutableSortedSet;
+
+/**
+ * Models a {@link DistributionFitter} with equally sized intervals.
+ *
+ * <p>The interval boundaries are defined by {@code width * i + offset} for {@code i} in {@code [0,
+ * numFiniteIntervals}.
+ *
+ * <p>For example, a {@link LinearFitter} with {@code numFiniteIntervals=2, width=10, offset=5}
+ * represents a histogram with intervals {@code (-inf, 5), [5, 15), [15, 25), [25, +inf)}.
+ */
+@AutoValue
+public abstract class LinearFitter implements DistributionFitter {
+
+  /**
+   * Create a new {@link LinearFitter}.
+   *
+   * @param numFiniteIntervals the number of intervals, excluding the underflow and overflow
+   *     intervals
+   * @param width the width of each interval
+   * @param offset the start value of the first interval
+   * @throws IllegalArgumentException if {@code numFiniteIntervals <= 0} or {@code width <= 0}
+   */
+  public static LinearFitter create(int numFiniteIntervals, double width, double offset) {
+    checkArgument(numFiniteIntervals > 0, "numFiniteIntervals must be greater than 0");
+    checkArgument(width > 0, "width must be greater than 0");
+    checkDouble(offset);
+
+    ImmutableSortedSet.Builder<Double> boundaries = ImmutableSortedSet.naturalOrder();
+
+    for (int i = 0; i < numFiniteIntervals + 1; i++) {
+      boundaries.add(width * i + offset);
+    }
+
+    return new AutoValue_LinearFitter(width, offset, boundaries.build());
+  }
+
+  public abstract double width();
+
+  public abstract double offset();
+
+  @Override
+  public abstract ImmutableSortedSet<Double> boundaries();
+}