adding comments to explain merge failure in segmentMetadata query

processing/src/main/java/io/druid/query/metadata/SegmentMetadataQueryQueryToolChest.java

@@ -149,6 +149,8 @@ public class SegmentMetadataQueryQueryToolChest extends QueryToolChest<SegmentAn
 
             List<Interval> newIntervals = null;
             if (query.hasInterval()) {
+              //List returned by arg1.getIntervals() is immutable, so a new list needs to
+              //be created.
               newIntervals = new ArrayList<>(arg1.getIntervals());
               newIntervals.addAll(arg2.getIntervals());
             }

processing/src/test/java/io/druid/query/metadata/SegmentMetadataQueryTest.java

@@ -164,7 +164,7 @@ public class SegmentMetadataQueryTest
             testQuery,
             Maps.newHashMap()
         ),
-        "failed SegmentMetadata bySegment query"
+        "failed SegmentMetadata merging query"
     );
     exec.shutdownNow();
   }