pubsub.go
func (p *PubSub) Subscribe(topic string, opts ...SubOpt) (*Subscription, error) {
	td := pb.TopicDescriptor{Name: &topic}

	return p.SubscribeByTopicDescriptor(&td, opts...)
}
func (p *PubSub) Subscribe(topic string, opts ...SubOpt) (*Subscription, error) {
	td := pb.TopicDescriptor{Name: &topic}

	return p.SubscribeByTopicDescriptor(&td, opts...)
}
// Subscribe returns a new Subscription for the given topic.
// Note that subscription is not an instanteneous operation. It may take some time
// before the subscription is processed by the pubsub main loop and propagated to our peers.