@Override
      public Result call(RegionObserver observer) throws IOException {
        return observer.preAppend(this, append, edit);
      }
Supports Coprocessor 'bypass'.
@param append append object
@param edit   The WALEdit object.
@return result to return to client if default operation should be bypassed, null otherwise
@throws IOException if an error occurred on the coprocessor
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.java