code: func (b *pubsubBroker) Publish(topic string, msg *broker.Message, opts ...broker.PublishOption) error {
  	if b.err != nil {
  		return b.err
  	}
  	t := b.topic(topic)
  	return t.Send(context.Background(), &pubsub.Message{Metadata: msg.Header, Body: msg.Body})
  }
docs: // Publish opens the topic if it hasn't been already, then publishes the message.