default void preMergeRegions(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    final RegionInfo[] regionsToMerge) throws IOException {
  }
Called before merge regions request.
@param ctx            coprocessor environment
@param regionsToMerge regions to be merged
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java