public Histogram getRequestTotalTimeHistogram() {
      return requestTotalTimeHistogram;
    }
Get the histogram object used for tracking request total time. This should be called only after corresponding
visit(Request request) method is invoked.
@return {@link Histogram} associated with tracking the request total times of the request.
ambry-server/src/main/java/com/github/ambry/server/AmbryRequests.java