Add BucketMetricValue interface (#24188)

Unlike other implementations of InternalNumericMetricsAggregation.SingleValue,
the InternalBucketMetricValue aggregation currently doesn't implement a
specialized interface that exposes the `keys()` method. This change adds this so
that clients can access the keys via the interface.

core/src/main/java/org/elasticsearch/search/aggregations/pipeline/bucketmetrics/BucketMetricValue.java

@@ -0,0 +1,27 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.search.aggregations.pipeline.bucketmetrics;
+
+import org.elasticsearch.search.aggregations.metrics.NumericMetricsAggregation;
+
+public interface BucketMetricValue extends NumericMetricsAggregation.SingleValue {
+
+    String[] keys();
+}

core/src/main/java/org/elasticsearch/search/aggregations/pipeline/bucketmetrics/InternalBucketMetricValue.java

@@ -31,7 +31,7 @@ import java.io.IOException;
 import java.util.List;
 import java.util.Map;
 
-public class InternalBucketMetricValue extends InternalNumericMetricsAggregation.SingleValue {
+public class InternalBucketMetricValue extends InternalNumericMetricsAggregation.SingleValue implements BucketMetricValue {
     public static final String NAME = "bucket_metric_value";
 
     private double value;
@@ -72,6 +72,7 @@ public class InternalBucketMetricValue extends InternalNumericMetricsAggregation
         return value;
     }
 
+    @Override
     public String[] keys() {
         return keys;
     }