Fold RestGetAllSettingsAction in RestGetSettingsAction (#30561)

We currently have a separate endpoint for retrieving settings from all indices. We introduced such endpoint when removing comma-separated feature parsing for GetIndicesAction. The RestGetAllSettingsAction duplicates the code to print out the response that we already have in GetSettingsResponse (since it became a ToXContentObject), and uses the get index API internally instead of the get settings API, but the response is the same, hence we can fold get all settings and get settings in a single API, which is what this commit does.

server/src/main/java/org/elasticsearch/action/ActionModule.java

@@ -253,7 +253,6 @@
 import org.elasticsearch.rest.action.admin.indices.RestGetAliasesAction;
 import org.elasticsearch.rest.action.admin.indices.RestGetAllAliasesAction;
 import org.elasticsearch.rest.action.admin.indices.RestGetAllMappingsAction;
-import org.elasticsearch.rest.action.admin.indices.RestGetAllSettingsAction;
 import org.elasticsearch.rest.action.admin.indices.RestGetFieldMappingAction;
 import org.elasticsearch.rest.action.admin.indices.RestGetIndexTemplateAction;
 import org.elasticsearch.rest.action.admin.indices.RestGetIndicesAction;
@@ -558,7 +557,6 @@ public void initRestHandlers(Supplier<DiscoveryNodes> nodesInCluster) {
 
         registerHandler.accept(new RestGetAllAliasesAction(settings, restController));
         registerHandler.accept(new RestGetAllMappingsAction(settings, restController));
-        registerHandler.accept(new RestGetAllSettingsAction(settings, restController, indexScopedSettings, settingsFilter));
         registerHandler.accept(new RestGetIndicesAction(settings, restController, indexScopedSettings, settingsFilter));
         registerHandler.accept(new RestIndicesStatsAction(settings, restController));
         registerHandler.accept(new RestIndicesSegmentsAction(settings, restController));

server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetSettingsAction.java

@@ -19,16 +19,12 @@
 
 package org.elasticsearch.rest.action.admin.indices;
 
-import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
-
 import org.elasticsearch.action.admin.indices.settings.get.GetSettingsRequest;
 import org.elasticsearch.action.admin.indices.settings.get.GetSettingsResponse;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.client.node.NodeClient;
 import org.elasticsearch.common.Strings;
-import org.elasticsearch.common.settings.IndexScopedSettings;
 import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.common.settings.SettingsFilter;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.BytesRestResponse;
@@ -46,6 +42,7 @@ public class RestGetSettingsAction extends BaseRestHandler {
 
     public RestGetSettingsAction(Settings settings, RestController controller) {
         super(settings);
+        controller.registerHandler(GET, "/_settings", this);
         controller.registerHandler(GET, "/_settings/{name}", this);
         controller.registerHandler(GET, "/{index}/_settings", this);
         controller.registerHandler(GET, "/{index}/_settings/{name}", this);