protected double getRegionLoadCost(Collection<BalancerRegionLoad> regionLoadList) {
    double cost = 0;
    for (BalancerRegionLoad rl : regionLoadList) {
      cost += getCostFromRl(rl);
    }
    return cost / regionLoadList.size();
  }
Base class the allows writing costs functions from rolling average of some number from
RegionLoad.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/CostFromRegionLoadFunction.java