@Override
      public void call(MasterObserver observer) throws IOException {
        observer.preMergeRegionsCommitAction(this, regionsToMerge, metaEntries);
      }
Invoked before merge regions operation writes the new region to hbase:meta
@param regionsToMerge the regions to merge
@param metaEntries    the meta entry
@param user           the user
hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterCoprocessorHost.java