Add method to create and register EventMetrics

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132346375

java/google/registry/monitoring/metrics/MetricRegistry.java

@@ -103,6 +103,33 @@ public interface MetricRegistry {
       String valueDisplayName,
       ImmutableSet<LabelDescriptor> labels);
 
+  /**
+   * Returns a new {@link EventMetric}.
+   *
+   * <p>This metric type is intended for recording aspects of an "event" -- things like latency or
+   * payload size.
+   *
+   * <p>The metric's values are {@link Distribution} instances which are updated via {@link
+   * EventMetric#record(double, String...)}.
+   *
+   * <p>The metric is thread-safe. The metric will be registered at the time of creation and
+   * collected for subsequent write intervals.
+   *
+   * @param name name of the metric. Should be in the form of '/foo/bar'.
+   * @param description human readable description of the metric.
+   * @param valueDisplayName human readable description of the metric's value type.
+   * @param labels list of the metric's labels.
+   * @param distributionFitter fit to apply to the underlying {@link Distribution} instances of this
+   *     metric.
+   * @throws IllegalStateException if a metric of the same name is already registered.
+   */
+  EventMetric newEventMetric(
+      String name,
+      String description,
+      String valueDisplayName,
+      ImmutableSet<LabelDescriptor> labels,
+      DistributionFitter distributionFitter);
+
   /**
    * Fetches a snapshot of the currently registered metrics
    *