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.

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

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

@@ -22,7 +22,7 @@ import org.junit.rules.ExpectedException;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
-/** Tests for implementations of {@link DistributionFitter}. */
+/** Unit tests for {@link ExponentialFitter}. */
 @RunWith(JUnit4.class)
 public class ExponentialFitterTest {