code: func (j *JSONFormatter) Format(e *log.Entry) ([]byte, error) {
  	if cursor := findFrame(); cursor != nil {
  		t := newTraceFromFrames(*cursor, nil)
  		new := e.WithFields(log.Fields{
  			FileField:     t.Loc(),
  			FunctionField: t.FuncName(),
  		})
  		new.Time = e.Time
  		new.Level = e.Level
  		new.Message = e.Message
  		e = new
  	}
  	return j.JSONFormatter.Format(e)
  }
docs: // Format implements logrus.Formatter interface