[ES|QL] Add aggregate metric double feature flag to its capability (elastic#121318) (elastic#121343)

AggregateMetricDouble should be behind a feature flag but on release
builds it was getting added when it should not have been. This commit
addresses that bug.

Author: limotova

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -20,6 +20,8 @@
 import java.util.Locale;
 import java.util.Set;
 
+import static org.elasticsearch.xpack.esql.core.plugin.EsqlCorePlugin.AGGREGATE_METRIC_DOUBLE_FEATURE_FLAG;
+
 /**
  * A {@link Set} of "capabilities" supported by the {@link RestEsqlQueryAction}
  * and {@link RestEsqlAsyncQueryAction} APIs. These are exposed over the
@@ -662,7 +664,7 @@ public enum Cap {
         /**
          * Support for aggregate_metric_double type
          */
-        AGGREGATE_METRIC_DOUBLE;
+        AGGREGATE_METRIC_DOUBLE(AGGREGATE_METRIC_DOUBLE_FEATURE_FLAG.isEnabled());
 
         private final boolean enabled;