default void postCompletedMergeRegionsAction(
    final ObserverContext<MasterCoprocessorEnvironment> ctx, final RegionInfo[] regionsToMerge,
    final RegionInfo mergedRegion) throws IOException {
  }
called after the regions merge.
@param ctx the environment to interact with the framework and master
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java