Add cluster UUID to Cluster Stats API response

docs/reference/cluster/stats.asciidoc

@@ -22,6 +22,7 @@ Will return, for example:
       "successful" : 1,
       "failed" : 0
    },
+   "cluster_uuid": "YjAvIhsCQ9CbjWZb2qJw3Q",
    "cluster_name": "elasticsearch",
    "timestamp": 1459427693515,
    "status": "green",

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.stats/10_basic.yml

@@ -29,3 +29,40 @@
   - is_true: nodes.fs
   - is_true: nodes.plugins
   - is_true: nodes.network_types
+
+---
+"get cluster stats returns cluster_uuid at the top level":
+  - skip:
+      version: " - 6.99.99"
+      reason:  "cluster stats including cluster_uuid at the top level is new in v6.5.0 and higher"
+
+  - do:
+      cluster.stats: {}
+
+  - is_true: cluster_uuid
+  - is_true: timestamp
+  - is_true: cluster_name
+  - match: {status: green}
+  - gte: { indices.count: 0}
+  - is_true: indices.docs
+  - is_true: indices.store
+  - is_true: indices.fielddata
+  - is_true: indices.query_cache
+  - is_true: indices.completion
+  - is_true: indices.segments
+  - gte: { nodes.count.total: 1}
+  - gte: { nodes.count.master: 1}
+  - gte: { nodes.count.data: 1}
+  - gte: { nodes.count.ingest: 0}
+  - gte: { nodes.count.coordinating_only: 0}
+  - is_true: nodes.os
+  - is_true: nodes.os.mem.total_in_bytes
+  - is_true: nodes.os.mem.free_in_bytes
+  - is_true: nodes.os.mem.used_in_bytes
+  - gte: { nodes.os.mem.free_percent: 0 }
+  - gte: { nodes.os.mem.used_percent: 0 }
+  - is_true: nodes.process
+  - is_true: nodes.jvm
+  - is_true: nodes.fs
+  - is_true: nodes.plugins
+  - is_true: nodes.network_types

server/src/main/java/org/elasticsearch/action/admin/cluster/stats/ClusterStatsResponse.java

@@ -40,15 +40,18 @@ public class ClusterStatsResponse extends BaseNodesResponse<ClusterStatsNodeResp
     ClusterStatsIndices indicesStats;
     ClusterHealthStatus status;
     long timestamp;
+    String clusterUUID;
 
     ClusterStatsResponse() {
     }
 
     public ClusterStatsResponse(long timestamp,
+                                String clusterUUID,
                                 ClusterName clusterName,
                                 List<ClusterStatsNodeResponse> nodes,
                                 List<FailedNodeException> failures) {
         super(clusterName, nodes, failures);
+        this.clusterUUID = clusterUUID;
         this.timestamp = timestamp;
         nodesStats = new ClusterStatsNodes(nodes);
         indicesStats = new ClusterStatsIndices(nodes);
@@ -61,6 +64,10 @@ public ClusterStatsResponse(long timestamp,
         }
     }
 
+    public String getClusterUUID() {
+        return this.clusterUUID;
+    }
+
     public long getTimestamp() {
         return this.timestamp;
     }
@@ -111,6 +118,7 @@ protected void writeNodesTo(StreamOutput out, List<ClusterStatsNodeResponse> nod
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.field("cluster_uuid", getClusterUUID());
         builder.field("timestamp", getTimestamp());
         if (status != null) {
             builder.field("status", status.name().toLowerCase(Locale.ROOT));

server/src/main/java/org/elasticsearch/action/admin/cluster/stats/TransportClusterStatsAction.java

@@ -74,6 +74,7 @@ protected ClusterStatsResponse newResponse(ClusterStatsRequest request,
                                                List<ClusterStatsNodeResponse> responses, List<FailedNodeException> failures) {
         return new ClusterStatsResponse(
             System.currentTimeMillis(),
+            clusterService.state().metaData().clusterUUID(),
             clusterService.getClusterName(),
             responses,
             failures);

x-pack/plugin/monitoring/src/test/java/org/elasticsearch/xpack/monitoring/collector/cluster/ClusterStatsMonitoringDocTests.java

@@ -302,6 +302,7 @@ public void testToXContent() throws IOException {
         when(mockNodeResponse.shardsStats()).thenReturn(new ShardStats[]{mockShardStats});
 
         final ClusterStatsResponse clusterStats = new ClusterStatsResponse(1451606400000L,
+                                                                            "_cluster",
                                                                             clusterName,
                                                                             singletonList(mockNodeResponse),
                                                                             emptyList());
@@ -353,6 +354,7 @@ public void testToXContent() throws IOException {
                     + (needToEnableTLS ? ",\"cluster_needs_tls\":true" : "")
                   + "},"
                   + "\"cluster_stats\":{"
+                    + "\"cluster_uuid\":\"_cluster\","
                     + "\"timestamp\":1451606400000,"
                     + "\"status\":\"red\","
                     + "\"indices\":{"