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

javatests/google/registry/monitoring/metrics/CustomFitterTest.java

@@ -0,0 +1,55 @@
+// 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.truth.Truth.assertThat;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.ImmutableSortedSet;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for {@link CustomFitter}. */
+@RunWith(JUnit4.class)
+public class CustomFitterTest {
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void testCreateCustomFitter_emptyBounds_throwsException() throws Exception {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("boundaries must not be empty");
+
+    CustomFitter.create(ImmutableSet.<Double>of());
+  }
+
+  @Test
+  public void testCreateCustomFitter_outOfOrderBounds_throwsException() throws Exception {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("boundaries must be sorted");
+
+    CustomFitter.create(ImmutableSet.of(2.0, 0.0));
+  }
+
+  @Test
+  public void testCreateCustomFitter_hasGivenBounds() {
+    CustomFitter fitter = CustomFitter.create(ImmutableSortedSet.of(1.0, 2.0));
+
+    assertThat(fitter.boundaries()).containsExactly(1.0, 2.0).inOrder();
+  }
+}