public void addToCounter(String counterName, long delta) {
    AtomicLong c = this.counters.get(counterName);
    if (c != null) {
      c.addAndGet(delta);
    }
  }
Returns {@link AtomicLong} instance for this counter name, null if counter does not exist.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/metrics/ServerSideScanMetrics.java