private void _registerAllocationInHostMap(String host, Map<String, AtomicInteger> hostMap) {
    AtomicInteger val = hostMap.get(host);
    if (val == null) {
      val = new AtomicInteger(0);
      hostMap.put(host, val);
    }
    val.incrementAndGet();
  }
Update DAG metrics caused by an incoming allocate Task call.
Updated metrics should use Atomics here to avoid using a lock on the caller.
@param requestedHosts
@param requestedRacks
llap-tez/src/java/org/apache/hadoop/hive/llap/tezplugins/scheduler/StatsPerDag.java