@Override
  public double getAverageTime() {
    return enoughStatistics()
      ? (double) sum(getNumberOfStatistics()) / getNumberOfStatistics()
      : (double) sum(getMostRecentPosition() + 1) / (getMostRecentPosition() + 1);
  }
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