code: public <R extends Request<T>> R addMarker(String tag, Object... args) {
          Marker marker = new Marker(tag,
                  Thread.currentThread().getId(),
                  Thread.currentThread().getName(),
                  System.nanoTime());
  
          synchronized (markerListeners) {
              for (RequestListener.MarkerListener markerListener : markerListeners) {
                  markerListener.onMarker(marker, args);
              }
          }
  
          if (logSlowRequests && requestBirthTime == 0) {
              requestBirthTime = System.nanoTime();
          }
  
          return (R) this;
      }
docs: Adds an event to this request's event log; for debugging.