@Override
  public void chargeSystemResourceUsage(QuotaName quotaName, double usageCost) {
    if (quotaName == QuotaName.READ_CAPACITY_UNIT) {
      feUsage.get().incrementRcu((long) Math.ceil(calculateFeUsageCost(quotaName, usageCost)));
    } else {
      feUsage.get().incrementWcu((long) Math.ceil(calculateFeUsageCost(quotaName, usageCost)));
    }
  }
Constructor for {@link AmbryCUQuotaSource}.
@param quotaConfig {@link QuotaConfig} object.
@param accountService {@link AccountService} object.
@param quotaMetrics {@link QuotaMetrics} object.
@param routerConfig {@link RouterConfig} object.
@throws IOException in case of any exception.
ambry-quota/src/main/java/com/github/ambry/quota/capacityunit/AmbryCUQuotaSource.java