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/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(