Allow dynamic updates for index.hidden setting (#52772)

This commit changes the `index.hidden` setting from being final to a
dynamic setting. While the setting being final allows for easier
reasoning about an index, making this setting update-able has more
benefits in that we can upgrade existing indices to be hidden and it
will enable future features that would dynamically make indices hidden.

server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetaData.java

@@ -262,7 +262,7 @@ public class IndexMetaData implements Diffable<IndexMetaData>, ToXContentFragmen
      * normal wildcard searches unless explicitly allowed
      */
     public static final Setting<Boolean> INDEX_HIDDEN_SETTING =
-        Setting.boolSetting(SETTING_INDEX_HIDDEN, false, Property.IndexScope, Property.Final);
+        Setting.boolSetting(SETTING_INDEX_HIDDEN, false, Property.Dynamic, Property.IndexScope);
 
     /**
      * an internal index format description, allowing us to find out if this index is upgraded or needs upgrading

server/src/test/java/org/elasticsearch/index/HiddenIndexIT.java

@@ -77,6 +77,15 @@ public class HiddenIndexIT extends ESIntegTestCase {
             .get();
         matchedHidden = Arrays.stream(searchResponse.getHits().getHits()).anyMatch(hit -> ".hidden-index".equals(hit.getIndex()));
         assertTrue(matchedHidden);
+
+        // make index not hidden
+        assertAcked(client().admin().indices().prepareUpdateSettings("hidden-index")
+            .setSettings(Settings.builder().put("index.hidden", false).build())
+            .get());
+        searchResponse =
+            client().prepareSearch(randomFrom("*", "_all", "h*", "*index")).setSize(1000).setQuery(QueryBuilders.matchAllQuery()).get();
+        matchedHidden = Arrays.stream(searchResponse.getHits().getHits()).anyMatch(hit -> "hidden-index".equals(hit.getIndex()));
+        assertTrue(matchedHidden);
     }
 
     public void testGlobalTemplatesDoNotApply() {

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/TransportResumeFollowAction.java

@@ -364,6 +364,7 @@ public class TransportResumeFollowAction extends TransportMasterNodeAction<Resum
             IndexMetaData.INDEX_BLOCKS_READ_ONLY_ALLOW_DELETE_SETTING,
             IndexMetaData.INDEX_PRIORITY_SETTING,
             IndexMetaData.SETTING_WAIT_FOR_ACTIVE_SHARDS,
+            IndexMetaData.INDEX_HIDDEN_SETTING,
             EnableAllocationDecider.INDEX_ROUTING_REBALANCE_ENABLE_SETTING,
             EnableAllocationDecider.INDEX_ROUTING_ALLOCATION_ENABLE_SETTING,
             ShardsLimitAllocationDecider.INDEX_TOTAL_SHARDS_PER_NODE_SETTING,