code: func ioOutputFormatJsonMessage(ld *LineData, b *bytes.Buffer) error {
  	msg, err := json.Marshal(ld.Message)
  	if err != nil {
  		return err
  	}
  
  	_, err = b.WriteString(string(msg))
  	return err
  }
docs: // Formatting function used to implement the %json:message% code.