Fix _cluster/state to always return cluster_uuid (#30656)

Since #30143, the Cluster State API should always returns the current
cluster_uuid in the response body, regardless of the metrics filters.

This is not exactly true as it is returned only if metadata metrics and
no specific indices are requested.

This commit fixes the behavior to always return the cluster_uuid and
add new test.

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.state/20_filtering.yml

@@ -163,12 +163,24 @@ setup:
       version:  " - 6.3.99"
       reason:   "cluster state including cluster_uuid at the top level is new in v6.4.0 and higher"
 
+  # Get the current cluster_uuid
+  - do:
+      cluster.state: {}
+  - set: { metadata.cluster_uuid : cluster_uuid }
+
   - do:
       cluster.state:
-        metric: [ master_node, version, metadata ]
+        metric: [ master_node, version ]
 
-  - is_true: cluster_uuid
+  - match: { cluster_uuid: $cluster_uuid }
   - is_true: master_node
   - is_true: version
   - is_true: state_uuid
-  - is_true: metadata
+
+  - do:
+      cluster.state:
+        metric: [ routing_table ]
+        index: testidx
+
+  - match: { cluster_uuid: $cluster_uuid }
+  - is_true: routing_table

server/src/main/java/org/elasticsearch/action/admin/cluster/state/TransportClusterStateAction.java

@@ -98,14 +98,11 @@ public class TransportClusterStateAction extends TransportMasterNodeReadAction<C
         if (request.blocks()) {
             builder.blocks(currentState.blocks());
         }
-        if (request.metaData()) {
-            MetaData.Builder mdBuilder;
-            if (request.indices().length == 0) {
-                mdBuilder = MetaData.builder(currentState.metaData());
-            } else {
-                mdBuilder = MetaData.builder();
-            }
 
+        MetaData.Builder mdBuilder = MetaData.builder();
+        mdBuilder.clusterUUID(currentState.metaData().clusterUUID());
+
+        if (request.metaData()) {
             if (request.indices().length > 0) {
                 String[] indices = indexNameExpressionResolver.concreteIndexNames(currentState, request);
                 for (String filteredIndex : indices) {
@@ -114,17 +111,19 @@ public class TransportClusterStateAction extends TransportMasterNodeReadAction<C
                         mdBuilder.put(indexMetaData, false);
                     }
                 }
+            } else {
+                mdBuilder = MetaData.builder(currentState.metaData());
             }
 
             // Filter our metadata that shouldn't be returned by API
-            for(ObjectObjectCursor<String, Custom> custom :  currentState.metaData().customs()) {
+            for(ObjectObjectCursor<String, Custom> custom : currentState.metaData().customs()) {
                 if(!custom.value.context().contains(MetaData.XContentContext.API)) {
                     mdBuilder.removeCustom(custom.key);
                 }
             }
-
-            builder.metaData(mdBuilder);
         }
+        builder.metaData(mdBuilder);
+
         if (request.customs()) {
             for (ObjectObjectCursor<String, ClusterState.Custom> custom : currentState.customs()) {
                 if (custom.value.isPrivate() == false) {