Add example of tracking user in onDisconnect callback

[SergeNarhi]

Add example of tracking user in onDisconnect callback. See #1597 and apollographql/subscriptions-transport-ws#95 for details.

[trevor-scheer]

Sorry for the delay on this @skanygin. We no longer recommend or document how to use subscriptions-transport-ws (in favor of graphql-ws).

If you find this use case particularly interesting, I'd be happy to review a PR which transposes the example you've provided over to the graphql-ws equivalent in our subscriptions docs:
https://github.com/apollographql/apollo-server/blob/main/docs/source/data/subscriptions.mdx

Worth mentioning the ctx object in our onConnect / onDisconnect example might sufficiently illustrate the fact that the context is shared between those functions. Or maybe the example could be expanded on just a bit to demonstrate your concept a bit more readily.