docs: private Stream<MemberId> getSubscriberNodes(String topicName) {
      InternalTopic topic = topics.get(topicName);
      if (topic == null) {
        return Stream.empty();
      }
      return topic.remoteSubscriptions().stream()
          .filter(s -> !s.isTombstone())
          .map(s -> s.memberId())
          .distinct();
    }