Add a way to introspect how many updates are currently being processed concurrently #3984
Description
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 :)