code: func WatchTopic(s string) registry.Option {
  	return func(o *registry.Options) {
  		if o.Context == nil {
  			o.Context = context.Background()
  		}
  		o.Context = context.WithValue(o.Context, watchTopicKey{}, s)
  	}
  }
docs: // WatchTopic allows to set a custom nats topic on which registries broadcast
  // changes (e.g. when services are added, updated or removed). Since we don't
  // have a central registry service, each service typically broadcasts in a
  // determined frequency on this topic.