Add method to create and register EventMetrics

-------------
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
    *

java/google/registry/monitoring/metrics/MetricRegistryImpl.java

@@ -42,6 +42,21 @@ public final class MetricRegistryImpl implements MetricRegistry {
     return INSTANCE;
   }
 
+  @Override
+  public EventMetric newEventMetric(
+      String name,
+      String description,
+      String valueDisplayName,
+      ImmutableSet<LabelDescriptor> labels,
+      DistributionFitter distributionFitter) {
+    EventMetric metric =
+        new EventMetric(name, description, valueDisplayName, distributionFitter, labels);
+    registerMetric(name, metric);
+    logger.info("Registered new event metric: " + name);
+
+    return metric;
+  }
+
   @Override
   @CanIgnoreReturnValue
   public <V> Metric<V> newGauge(

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

@@ -138,6 +138,29 @@ public class MetricRegistryImplTest {
                 ImmutableSet.of(label)));
   }
 
+  @Test
+  public void testNewEventMetric_createsEventMetric() {
+    DistributionFitter fitter = CustomFitter.create(ImmutableSet.of(0.0));
+    EventMetric testMetric =
+        MetricRegistryImpl.getDefault()
+            .newEventMetric(
+                "/test_metric",
+                "test_description",
+                "test_valuedisplayname",
+                ImmutableSet.of(label),
+                fitter);
+
+    assertThat(testMetric.getValueClass()).isSameAs(Distribution.class);
+    assertThat(testMetric.getMetricSchema())
+        .isEqualTo(
+            MetricSchema.create(
+                "/test_metric",
+                "test_description",
+                "test_valuedisplayname",
+                Kind.CUMULATIVE,
+                ImmutableSet.of(label)));
+  }
+
   @Test
   public void testRegister_duplicateMetric_throwsException() {
     SettableMetric<Boolean> testMetric =