private void onRequestDequeue(AsyncRequestInfo requestInfo) {
    queuedRequestCount.decrementAndGet();
    metrics.requestDequeuingRate.mark();
    long processingDelay = requestInfo.getProcessingDelay();
    requestInfo.restRequest.getMetricsTracker().scalingMetricsTracker.addToRequestProcessingWaitTime(processingDelay);
  }
Tracks required metrics once a {@link AsyncRequestInfo} is dequeued.
@param requestInfo the {@link AsyncRequestInfo} that was just dequeued.
ambry-rest/src/main/java/com/github/ambry/rest/AsyncRequestResponseHandler.java