code: func (r *Request) BackOff(manager BackoffManager) *Request {
  	if manager == nil {
  		r.backoffMgr = &NoBackoff{}
  		return r
  	}
  
  	r.backoffMgr = manager
  	return r
  }
docs: // BackOff sets the request's backoff manager to the one specified,
  // or defaults to the stub implementation if nil is provided