Add a Fibonacci fitter for metrics bucketing

A Fibonacci fitter is useful in situations where you want more precision on the
low end than an ExponentialFitter with exponent base 2 provides without the
hassle of dealing with non-integer boundaries, such as would be created by an
exponential fitter with a base of less than 2. Fibonacci fitters are ideal for
integer metrics that are bounded across a certain range, e.g. integers between 1
and 1,000.

This also cleans up some unit test comments.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=156773367

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

@@ -0,0 +1,83 @@
+// Copyright 2017 The Nomulus 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 static org.junit.Assert.fail;
+
+import com.google.common.collect.ImmutableList;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link FibonacciFitter}. */
+@RunWith(JUnit4.class)
+public class FibonacciFitterTest {
+
+  @Test
+  public void testCreate_maxBucketSizeNegative_throwsException() {
+    try {
+      FibonacciFitter.create(-1);
+      fail("Expected exception");
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessageThat().isEqualTo("maxBucketSize must be greater than 0");
+    }
+  }
+
+  @Test
+  public void testCreate_maxBucketSizeZero_throwsException() {
+    try {
+      FibonacciFitter.create(0);
+      fail("Expected exception");
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessageThat().isEqualTo("maxBucketSize must be greater than 0");
+    }
+  }
+
+  @Test
+  public void testCreate_maxBucketSizeOne_createsTwoBoundaries() {
+    assertThat(FibonacciFitter.create(1).boundaries()).containsExactly(0.0, 1.0).inOrder();
+  }
+
+  @Test
+  public void testCreate_maxBucketSizeTwo_createsThreeBoundaries() {
+    assertThat(FibonacciFitter.create(2).boundaries()).containsExactly(0.0, 1.0, 2.0).inOrder();
+  }
+
+  @Test
+  public void testCreate_maxBucketSizeThree_createsFourBoundaries() {
+    assertThat(FibonacciFitter.create(3).boundaries())
+        .containsExactly(0.0, 1.0, 2.0, 3.0)
+        .inOrder();
+  }
+
+  @Test
+  public void testCreate_maxBucketSizeFour_createsFourBoundaries() {
+    assertThat(FibonacciFitter.create(4).boundaries())
+        .containsExactly(0.0, 1.0, 2.0, 3.0)
+        .inOrder();
+  }
+
+  @Test
+  public void testCreate_maxBucketSizeLarge_createsFibonacciSequenceBoundaries() {
+    ImmutableList<Double> expectedBoundaries =
+        ImmutableList.of(
+            0.0, 1.0, 2.0, 3.0, 5.0, 8.0, 13.0, 21.0, 34.0, 55.0, 89.0, 144.0, 233.0, 377.0, 610.0,
+            987.0);
+    assertThat(FibonacciFitter.create(1000).boundaries())
+        .containsExactlyElementsIn(expectedBoundaries)
+        .inOrder();
+  }
+}