Serialize metas and reference attributes to a JSON string.

        Keyword Arguments:
            indent (int): Space indentation, default to ``4``.

        Returns:
            string: JSON datas.

def to_json(self, indent=4):
        """
        Serialize metas and reference attributes to a JSON string.

        Keyword Arguments:
            indent (int): Space indentation, default to ``4``.

        Returns:
            string: JSON datas.
        """
        agregate = {
            'metas': self.metas,
        }

        agregate.update({k: getattr(self, k) for k in self._rule_attrs})

        return json.dumps(agregate, indent=indent)