Monitoring should wait with collecting data when cluster service is started. (#48277)

Otherwise integration tests may fail if the monitoring interval is low:
```
[2019-10-21T09:57:25,527][ERROR][o.e.b.ElasticsearchUncaughtExceptionHandler] [integTest-0] fatal error in thread [elasticsearch[integTest-0][generic][T#4]], exiting
java.lang.AssertionError: initial cluster state not set yet
        at org.elasticsearch.cluster.service.ClusterApplierService.state(ClusterApplierService.java:208) ~[elasticsearch-7.6.0-SNAPSHOT.jar:7.6.0-SNAPSHOT]
        at org.elasticsearch.cluster.service.ClusterService.state(ClusterService.java:125) ~[elasticsearch-7.6.0-SNAPSHOT.jar:7.6.0-SNAPSHOT]
        at org.elasticsearch.xpack.monitoring.MonitoringService$MonitoringExecution$1.doRun(MonitoringService.java:231) ~[?:?]
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) ~[elasticsearch-7.6.0-SNAPSHOT.jar:7.6.0-SNAPSHOT]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) ~[?:?]
        at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
        at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:703) ~[elasticsearch-7.6.0-SNAPSHOT.jar:7.6.0-SNAPSHOT]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
        at java.lang.Thread.run(Thread.java:835) [?:?]
```

I ran into this when lowering the monitoring interval when investigating
enrich monitoring test: #48258

Author: martijnvg

x-pack/plugin/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/MonitoringService.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.component.AbstractLifecycleComponent;
+import org.elasticsearch.common.component.Lifecycle;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
@@ -218,6 +219,11 @@ public void doRun() {
                 return;
             }
 
+            if (clusterService.lifecycleState() != Lifecycle.State.STARTED) {
+                logger.debug("cluster service not started");
+                return;
+            }
+
             if (semaphore.tryAcquire() == false) {
                 logger.debug("monitoring execution is skipped until previous execution terminated");
                 return;

x-pack/plugin/monitoring/src/test/java/org/elasticsearch/xpack/monitoring/MonitoringServiceTests.java

@@ -8,6 +8,7 @@
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.component.Lifecycle;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.license.XPackLicenseState;
@@ -57,6 +58,7 @@ protected XPackLicenseState getLicenseState() {
         clusterSettings = new ClusterSettings(Settings.EMPTY, new HashSet<>(monitoring.getSettings()));
         when(clusterService.getClusterSettings()).thenReturn(clusterSettings);
         when(clusterService.state()).thenReturn(mock(ClusterState.class));
+        when(clusterService.lifecycleState()).thenReturn(Lifecycle.State.STARTED);
     }
 
     @After