Strict level parsing for indices stats

A previous commit added strict level parsing for the node stats API, but
that commit missed adding the same for the indices stats API. This
commit rectifies this miss.

Relates #21577

core/src/main/java/org/elasticsearch/action/admin/indices/stats/IndicesStatsResponse.java

@@ -152,12 +152,14 @@ public class IndicesStatsResponse extends BroadcastResponse implements ToXConten
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
-        String level = params.param("level", "indices");
-        boolean isLevelValid = "indices".equalsIgnoreCase(level) || "shards".equalsIgnoreCase(level) || "cluster".equalsIgnoreCase(level);
+        final String level = params.param("level", "indices");
+        final boolean isLevelValid =
+            "cluster".equalsIgnoreCase(level) || "indices".equalsIgnoreCase(level) || "shards".equalsIgnoreCase(level);
         if (!isLevelValid) {
-            return builder;
+            throw new IllegalArgumentException("level parameter must be one of [cluster] or [indices] or [shards] but was [" + level + "]");
         }
 
+
         builder.startObject("_all");
 
         builder.startObject("primaries");

core/src/test/java/org/elasticsearch/action/admin/indices/stats/IndicesStatsResponseTests.java

@@ -0,0 +1,43 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.indices.stats;
+
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.test.ESTestCase;
+
+import java.util.Collections;
+
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.hamcrest.object.HasToString.hasToString;
+
+
+public class IndicesStatsResponseTests extends ESTestCase {
+
+    public void testInvalidLevel() {
+        final IndicesStatsResponse response = new IndicesStatsResponse();
+        final String level = randomAsciiOfLength(16);
+        final ToXContent.Params params = new ToXContent.MapParams(Collections.singletonMap("level", level));
+        final IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> response.toXContent(null, params));
+        assertThat(
+            e,
+            hasToString(containsString("level parameter must be one of [cluster] or [indices] or [shards] but was [" + level + "]")));
+    }
+
+}