[FEATURE] Add a way to introspect how many updates are currently being processed concurrently

[clot27]

What kind of feature are you missing? Where do you notice a shortcoming of PTB?

As of now, we can't know how many updates are being processed while using BUP, it would be good to have that for debugging purposes.

Describe the solution you'd like

See #3654 (comment)

Describe alternatives you've considered

Currently, you can use app.update_processor._semaphore._value to get those (I suppose).

Additional context

Was originally suggested by harshil in dev chat IIRC :)