Format the message into JSON expected by fluentd.

        :type record: :class:`~logging.LogRecord`
        :param record: the log record

        :rtype: str
        :returns: A JSON string formatted for GKE fluentd.

def format(self, record):
        """Format the message into JSON expected by fluentd.

        :type record: :class:`~logging.LogRecord`
        :param record: the log record

        :rtype: str
        :returns: A JSON string formatted for GKE fluentd.
        """
        message = super(ContainerEngineHandler, self).format(record)
        return format_stackdriver_json(record, message)