Add mpsc::Sender strong and weak counts, similar to Arc counts

[vasilakisfil]

Is your feature request related to a problem? Please describe.
The tokio actor pattern described in @Darksonn's blog post is very useful and pervasive. With #4023 we also have WeakSender, automatically making Sender a "strong" Sender. But if we have strong and weak senders, shouldn't we also expose the relevant counts as well?

Describe the solution you'd like
Just how Arc does with strong and weak counts, tokio's mpsc::Sender should expose similar counts.

Describe alternatives you've considered
We could probably wrap these types using newtype pattern, implement our custom clone and achieve the same result, if we desperately needed that. But it would be nice if official API offered these counts

Additional context
We have extended Alice's initial actor pattern with automatic actor termination when all relevant handles get dropped, graceful shutdown and on some actors even, we provide "weak" handles, implemented on top of mpsc::WeakSender, offering similar functionality (doesn't prevent the actor to get dropped even if there are such "weak" handles around). It would be really helpful to have the counts as we could provide better API guarantees on graceful shutdown. Specifically, we wouldn't allow gracefully terminating an actor, even if a handle requests that, if other "strong" handles still exist.

[Darksonn]

I'm not opposed to adding this.