Join the identifier tuple with periods ".", combine the arbitrary tags
        with the base tags and the identifier tag, convert tags to "tag=value"
        format, and then join everything with ",".

def _join(self, identifier, instance=None, tags=None):
        """
        Join the identifier tuple with periods ".", combine the arbitrary tags
        with the base tags and the identifier tag, convert tags to "tag=value"
        format, and then join everything with ",".
        """
        tag_list = []
        if tags is not None:
            tag_list.extend(tags.items())
        tag_list.extend(self._base_tags)
        return ".".join(identifier) + "," + ",".join(
            "{}={}".format(k, v)
            for k, v in tag_list
        )