@Override
  public void postLogRoll(Path oldPath, Path newPath) {
    // oldPath can be null if this is the first time we created a wal
    // Also newPath can be equal to oldPath if AbstractFSWAL#replaceWriter fails
    if (newPath != oldPath) {
      source.incrementSuccessfulLogRolls();
    }
  }
Class used to push numbers about the WAL into the metrics subsystem. This will take a single
function call and turn it into multiple manipulations of the hadoop metrics system.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/MetricsWAL.java