code: func LogCallback(c context.Context, opname string) Callback {
  	return func(err error, delay time.Duration) {
  		logging.Fields{
  			logging.ErrorKey: err,
  			"opname":         opname,
  			"delay":          delay,
  		}.Warningf(c, "operation failed transiently")
  	}
  }
docs: // LogCallback builds a Callback which logs a Warning with the opname, error
  // and delay.