@Override
      public void onCompletion(Void result, Exception exception) {
        slots.add(true);
        logger.trace("Slot added");
      }
Reports exceptions if any and adds a slot so that it can be reused for another request.
@param result The result of the request. This would be non null when the request executed successfully
@param exception The exception that was reported on execution of the request
ambry-tools/src/main/java/com/github/ambry/tools/perf/rest/PerfNioServer.java