@Override
  protected void updateMostRecentTime(long elapsed) {
    int index = moveForwardMostRecentPosition();
    lastN[index] = elapsed;
  }
Instead of calculate a whole time average, this class focus on the last N. The last N is stored
in a circle array.
hbase-common/src/main/java/org/apache/hadoop/hbase/util/WindowMovingAverage.java