default void preMergeRegionsCommitAction(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    final RegionInfo[] regionsToMerge, @MetaMutationAnnotation List<Mutation> metaEntries)
    throws IOException {
  }
This will be called before update META step as part of regions merge transaction.
@param ctx         the environment to interact with the framework and master
@param metaEntries mutations to execute on hbase:meta atomically with regions merge updates.
                   Any puts or deletes to execute on hbase:meta can be added to the mutations.
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java