@RestrictedApi(explanation = "Should only be called in tests", link = "",
      allowedOnPath = ".*/src/test/.*")
  FastLongHistogram getHistogram() {
    return histogram;
  }
A tool to evaluating the lag between primary replica and secondary replica.
<p/>
It simply adds a row to the primary replica, and then check how long before we can read it from
the secondary replica.
hbase-server/src/test/java/org/apache/hadoop/hbase/RegionReplicationLagEvaluation.java