@Override
  public double getAverageTime() {
    if (!enoughStatistics()) {
      return super.getAverageTime();
    }
    return currentAverage;
  }
EMA is similar to {@link WeightedMovingAverage} in weighted, but the weighting factor decrease
exponentially. It brings benefits that it is more sensitive, and can see the trends easily.
hbase-common/src/main/java/org/apache/hadoop/hbase/util/ExponentialMovingAverage.java