fix debug output in TransportGetDataFrameAnalyticsStatsAction

Author: jan-elastic

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

@@ -61,6 +61,7 @@
 import org.elasticsearch.xpack.ml.utils.persistence.MlParserUtils;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
@@ -278,7 +279,7 @@ private void searchStats(DataFrameAnalyticsConfig config, TaskId parentTaskId, A
                             () -> format(
                                 "[%s] Item failure encountered during multi search for request [indices=%s, source=%s]: %s",
                                 config.getId(),
-                                itemRequest.indices(),
+                                Arrays.toString(itemRequest.indices()),
                                 itemRequest.source(),
                                 itemResponse.getFailureMessage()
                             ),