[ML] reduce the logging spam for the ml initializer for annotation index creation (#80149)

It is possible that the annotation index is in an unsupported state. Consequently, the same exception is thrown whenever the MlInitializationService attempts to create the index.

Logging all those failures at the error level doesn't make sense. This commit only logs the first seen (of duplicate errors) as error and the rest of debug.

Whenever the error changes, it is logged.

closes #65217

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlInitializationService.java

@@ -54,6 +54,7 @@ public class MlInitializationService implements ClusterStateListener {
     private final ThreadPool threadPool;
     private final AtomicBoolean isIndexCreationInProgress = new AtomicBoolean(false);
     private final AtomicBoolean mlInternalIndicesHidden = new AtomicBoolean(false);
+    private volatile String previousException;
 
     private final MlDailyMaintenanceService mlDailyMaintenanceService;
 
@@ -143,8 +144,13 @@ public class MlInitializationService implements ClusterStateListener {
                 event.state(),
                 MasterNodeRequest.DEFAULT_MASTER_NODE_TIMEOUT,
                 ActionListener.wrap(r -> isIndexCreationInProgress.set(false), e -> {
+                    if (e.getMessage().equals(previousException)) {
+                        logger.debug("Error creating ML annotations index or aliases", e);
+                    } else {
+                        previousException = e.getMessage();
+                        logger.error("Error creating ML annotations index or aliases", e);
+                    }
                     isIndexCreationInProgress.set(false);
-                    logger.error("Error creating ML annotations index or aliases", e);
                 })
             );
         }