code: def _join(self, identifier, instance=None, tags=None):
          
          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
          )
docs: 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 ",".