Replace 'discovered_master' with 'discovered_cluster_manager' in 'GET Cat Health' API (#2438)

* Replace 'discovered_master' with 'discovered_cluster_manager' in the table header of `GET Cat Health` API, to promote inclusive language.
* Add `dcm` as the alias for the table header `discovered_cluster_manager`
* Add `discovered_master` as the alias for the table header `discovered_cluster_manager`, for keeping compatibility when using `GET _cat/nodes?v&h=discovered_master` to show the specific column only.
* Add YAML rest test

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

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

@@ -1,11 +1,19 @@
 ---
-"Help":
+"Help - before 2.0.0":
   - skip:
-        version: " - 7.10.99"
-        reason: "discovered_master added in OpenSearch 1.0.0"
+      version: " - 7.10.99 , 2.0.0 - "
+      reason: "discovered_master added in OpenSearch 1.0.0, and renamed to discovered_cluster_manager in 2.0.0"
+      features: node_selector
   - do:
       cat.health:
         help: true
+      node_selector:
+        # Only send request to nodes in <2.0 versions, especially during ':qa:mixed-cluster:v1.x.x#mixedClusterTest'.
+        # Because YAML REST test takes the minimum OpenSearch version in the cluster to apply the filter in 'skip' section, 
+        # see OpenSearchClientYamlSuiteTestCase#initAndResetContext() for detail.
+        # During 'mixedClusterTest', the cluster can be mixed with nodes in 1.x and 2.x versions, 
+        # so node_selector is required, and only filtering version in 'skip' is not enough.
+        version: "1.0.0 - 1.4.99"
 
   - match:
       $body: |
@@ -27,6 +35,34 @@
 
                $/
 
+---
+"Help":
+  - skip:
+      version: " - 1.4.99"
+      reason: "discovered_cluster_manager is added in OpenSearch 2.0.0"
+  - do:
+      cat.health:
+        help: true
+
+  - match:
+      $body: |
+               /^  epoch         .+ \n
+                   timestamp     .+ \n
+                   cluster       .+ \n
+                   status        .+ \n
+                   node.total    .+ \n
+                   node.data     .+ \n
+                   discovered_cluster_manager .+ \n
+                   shards        .+ \n
+                   pri           .+ \n
+                   relo          .+ \n
+                   init          .+ \n
+                   unassign      .+ \n
+                   pending_tasks .+ \n
+                   max_task_wait_time .+ \n
+                   active_shards_percent .+ \n
+      
+               $/
 
 ---
 "Empty cluster":

server/src/main/java/org/opensearch/rest/action/cat/RestHealthAction.java

@@ -88,7 +88,12 @@ public class RestHealthAction extends AbstractCatAction {
         t.addCell("status", "alias:st;desc:health status");
         t.addCell("node.total", "alias:nt,nodeTotal;text-align:right;desc:total number of nodes");
         t.addCell("node.data", "alias:nd,nodeData;text-align:right;desc:number of nodes that can store data");
-        t.addCell("discovered_master", "alias:dm;text-align:right;desc:discovered master");
+        // TODO: Remove the header alias 'discovered_master', after removing MASTER_ROLE.
+        // The alias 'discovered_master' is added for compatibility when using request parameter 'h=discovered_master'.
+        t.addCell(
+            "discovered_cluster_manager",
+            "alias:dcm,dm,discovered_master;text-align:right;desc:cluster manager is discovered or not"
+        );
         t.addCell("shards", "alias:t,sh,shards.total,shardsTotal;text-align:right;desc:total number of shards");
         t.addCell("pri", "alias:p,shards.primary,shardsPrimary;text-align:right;desc:number of primary shards");
         t.addCell("relo", "alias:r,shards.relocating,shardsRelocating;text-align:right;desc:number of relocating nodes");