[Test] mute MonitoringIT#testMonitoringService

The MonitoringIT#testMonitoringService test fails occasionally in CI
due to a failed wait for documents condition, so this commit adds an
awaits fix.

See elastic/x-pack-elasticsearch#4150

Original commit: elastic/x-pack-elasticsearch@c9415cba63

plugin/monitoring/src/test/java/org/elasticsearch/xpack/monitoring/integration/MonitoringIT.java

@@ -187,6 +187,7 @@ public class MonitoringIT extends ESSingleNodeTestCase {
      * This test waits for the monitoring service to collect monitoring documents and then checks that all expected documents
      * have been indexed with the expected information.
      */
+    @AwaitsFix(bugUrl = "https://github.com/elastic/x-pack-elasticsearch/issues/4150")
     @SuppressWarnings("unchecked")
     public void testMonitoringService() throws Exception {
         final boolean createAPMIndex = randomBoolean();