public long addToRequestProcessingTime(long delta) {
      return requestProcessingTimeInMs.addAndGet(delta);
    }
Adds to the time taken to process a request at the scaling layer.
@param delta the time taken in ms to do the current piece of processing at the scaling layer for the request.
@return the total time taken in ms to process this request at the scaling layer, including the current piece, at
         this moment.
ambry-api/src/main/java/com/github/ambry/rest/RestRequestMetricsTracker.java