Finer grained logging and metrics for split transactions

A split transaction is a complex orchestration of activity between the RegionServer, Master, ZooKeeper, and HDFS NameNode. We have some visibility into the time taken by various phases of the split transaction in the logs. We will see "Starting split of region $PARENT" before the transaction begins, before the parent is offlined. Later we will see "Opening $DAUGHTER" as one of the last steps in the transaction, this is after the parent has been flushed, offlined, and closed. Finally ""Region split, hbase:meta updated, and report to master ... Split took $TIME" after all steps are complete and including the total running time of the transaction.

For debugging the cause(s) of long running split transactions it would be useful to know the distribution of time spent in all of the phases of the split transaction.