@Override
  protected double getCostFromRl(BalancerRegionLoad rl) {
    return rl.getWriteRequestsCount();
  }
Compute the cost of total number of write requests. The more unbalanced the higher the computed
cost will be. This uses a rolling average of regionload.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/WriteRequestCostFunction.java