How to attach the pubsub.subscribe, unsubscribe function to apollo server?

- [ ] has-reproduction
- [x] question
- [ ] blocking
- [ ] good first issue

I am using apollo server subscription, and I set up the subscription with the regular pubsub in this package.
use the pubsub.asyncIterator for subscriptions, then use the same pubsub.publish(xxx).

pubsub also provide an subscribe and unsubscribe function which keep track of count/clients etc. But these two functions are not called if I just setup like this. I understand these are more for custome behavior and such. But my main goal is to know how many clients are subscribed to my channel, and the parameters when they subscribed, right before I call the pubsub.publish function. What is the best way to do it?

Start by reviewing the package API points named in the issue: pubsub.asyncIterator, publish, subscribe, and unsubscribe. Clarify how subscription clients and their parameters can be observed, and document the supported approach before publishing to a channel.

Written by the indexing model from the issue text.