public long addToResponseProcessingWaitTime(long delta) {
      return responseProcessingWaitTimeInMs.addAndGet(delta);
    }
Adds to the scaling layer processing wait time of a response.
@param delta the time in ms a response has spent waiting to be processed at the scaling layer.
@return the total time in ms this response has spent waiting to be processed at the scaling layer at this moment.
ambry-api/src/main/java/com/github/ambry/rest/RestRequestMetricsTracker.java