Expose search.throttled on _cat/indices

Today it's very difficult to see which indices are frozen or rather
throttled via the commonly used monitoring APIs. This change adds
a cell to the `_cat/indices` API to render if an index is `search.throttled`

Relates to elastic#34352

docs/reference/frozen-indices.asciidoc

@@ -64,4 +64,25 @@ The default value for `pre_filter_shard_size` is `128` but it's recommended to s
 significant overhead associated with this pre-filter phase.
 ================================
 
+== Monitoring frozen indices
+
+Frozen indices are ordinary indices that use search throttling and a memory efficient shard implementation. In order to monitor / filter
+frozen indices in APIs like the <<cat-indices>> they must be filtered by their `throttled` property since frozen indices are not a core
+concept.
+
+[source,js]
+--------------------------------------------------
+GET /_cat/indices/twitter?v&h=i,sth
+--------------------------------------------------
+// CONSOLE
+// TEST[s/^/PUT twitter\nPOST twitter\/_freeze\n/]
+
+The response looks like:
+
+[source,txt]
+--------------------------------------------------
+i         sth
+twitter   true
+--------------------------------------------------
+// TESTRESPONSE[_cat]
 

server/src/main/java/org/elasticsearch/rest/action/cat/RestIndicesAction.java

@@ -41,6 +41,7 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.time.DateFormatters;
 import org.elasticsearch.index.Index;
+import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.rest.RestController;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.RestResponse;
@@ -335,6 +336,8 @@ protected Table getTableWithHeader(final RestRequest request) {
         table.addCell("memory.total", "sibling:pri;alias:tm,memoryTotal;default:false;text-align:right;desc:total used memory");
         table.addCell("pri.memory.total", "default:false;text-align:right;desc:total user memory");
 
+        table.addCell("search.throttled", "alias:sth;default:false;desc:indicates if the index is search throttled");
+
         table.endHeaders();
         return table;
     }
@@ -357,6 +360,7 @@ Table buildTable(RestRequest request, Index[] indices, ClusterHealthResponse res
             IndexStats indexStats = stats.getIndices().get(indexName);
             IndexMetaData indexMetaData = indexMetaDatas.getIndices().get(indexName);
             IndexMetaData.State state = indexMetaData.getState();
+            boolean searchThrottled = IndexSettings.INDEX_SEARCH_THROTTLED.get(indexMetaData.getSettings());
 
             if (status != null) {
                 if (state == IndexMetaData.State.CLOSE ||
@@ -558,6 +562,8 @@ Table buildTable(RestRequest request, Index[] indices, ClusterHealthResponse res
             table.addCell(indexStats == null ? null : indexStats.getTotal().getTotalMemory());
             table.addCell(indexStats == null ? null : indexStats.getPrimaries().getTotalMemory());
 
+            table.addCell(searchThrottled);
+
             table.endRow();
         }