Added cluster feature

Author: Mikep86

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/RankRRFFeatures.java

@@ -10,6 +10,7 @@
 import org.elasticsearch.features.FeatureSpecification;
 import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.xpack.rank.linear.LinearRetrieverBuilder;
+import org.elasticsearch.xpack.rank.rrf.RRFRetrieverBuilder;
 
 import java.util.Set;
 
@@ -34,7 +35,8 @@ public Set<NodeFeature> getTestFeatures() {
             LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX,
             LINEAR_RETRIEVER_L2_NORM,
             LINEAR_RETRIEVER_MINSCORE_FIX,
-            LinearRetrieverBuilder.MULTI_FIELDS_QUERY_FORMAT_SUPPORT
+            LinearRetrieverBuilder.MULTI_FIELDS_QUERY_FORMAT_SUPPORT,
+            RRFRetrieverBuilder.MULTI_FIELDS_QUERY_FORMAT_SUPPORT
         );
     }
 }

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java

@@ -11,6 +11,7 @@
 import org.elasticsearch.action.ActionRequestValidationException;
 import org.elasticsearch.action.ResolvedIndices;
 import org.elasticsearch.common.util.Maps;
+import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.index.query.MatchNoneQueryBuilder;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryRewriteContext;
@@ -46,6 +47,9 @@
  * formula.
  */
 public final class RRFRetrieverBuilder extends CompoundRetrieverBuilder<RRFRetrieverBuilder> {
+    public static final NodeFeature MULTI_FIELDS_QUERY_FORMAT_SUPPORT = new NodeFeature(
+        "rrf_retriever.multi_fields_query_format_support"
+    );
 
     public static final String NAME = "rrf";
 
@@ -218,6 +222,7 @@ protected RetrieverBuilder doRewrite(QueryRewriteContext ctx) {
 
         ResolvedIndices resolvedIndices = ctx.getResolvedIndices();
         if (resolvedIndices != null && query != null) {
+            // TODO: Refactor duplicate code
             // Using the multi-fields query format
             var localIndicesMetadata = resolvedIndices.getConcreteLocalIndicesMetadata();
             if (localIndicesMetadata.size() > 1) {