docs: def add_topic(self, topic):
          """Add a topic to the list of topics tracked via metadata.
  
          Arguments:
              topic (str): topic to track
  
          Returns:
              Future: resolves after metadata request/response
          """
          if topic in self._topics:
              return Future().success(set(self._topics))
  
          self._topics.add(topic)
          return self.cluster.request_update()