Add a field discovered_cluster_manager in get cluster health api

Signed-off-by: Tianli Feng <ftianli@amazon.com>

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

@@ -275,3 +275,16 @@
       cluster.health:
         index: index-2
   - match:     { status:     yellow }
+
+---
+"Get cluster health has same value for discovered_master and discovered_cluster_manager":
+  - skip:
+      version: " - 1.4.99"
+      reason: "discovered_cluster_manager is added in 2.0.0"
+  - do:
+      cluster.health: {}
+  - set:
+      discovered_master: discovered_or_not
+
+  - match:     { discovered_master:          $discovered_or_not }
+  - match:     { discovered_cluster_manager: $discovered_or_not }

server/src/main/java/org/opensearch/action/admin/cluster/health/ClusterHealthResponse.java

@@ -67,6 +67,7 @@ public class ClusterHealthResponse extends ActionResponse implements StatusToXCo
     private static final String NUMBER_OF_NODES = "number_of_nodes";
     private static final String NUMBER_OF_DATA_NODES = "number_of_data_nodes";
     private static final String DISCOVERED_MASTER = "discovered_master";
+    private static final String DISCOVERED_CLUSTER_MANAGER = "discovered_cluster_manager";
     private static final String NUMBER_OF_PENDING_TASKS = "number_of_pending_tasks";
     private static final String NUMBER_OF_IN_FLIGHT_FETCH = "number_of_in_flight_fetch";
     private static final String DELAYED_UNASSIGNED_SHARDS = "delayed_unassigned_shards";
@@ -376,7 +377,8 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         builder.field(TIMED_OUT, isTimedOut());
         builder.field(NUMBER_OF_NODES, getNumberOfNodes());
         builder.field(NUMBER_OF_DATA_NODES, getNumberOfDataNodes());
-        builder.field(DISCOVERED_MASTER, hasDiscoveredMaster());
+        builder.field(DISCOVERED_MASTER, hasDiscoveredMaster());  // the field will be removed in a future major version
+        builder.field(DISCOVERED_CLUSTER_MANAGER, hasDiscoveredMaster());
         builder.field(ACTIVE_PRIMARY_SHARDS, getActivePrimaryShards());
         builder.field(ACTIVE_SHARDS, getActiveShards());
         builder.field(RELOCATING_SHARDS, getRelocatingShards());