Add .len() method to UnboundedSender

[tqwewe]

Is your feature request related to a problem? Please describe.
The number of buffered messages can be derived from a bounded mpsc channel sender with len = tx.max_capacity() - tx.capacity(). However there's no methods available to get the number of buffered messages from an unbounded mpsc channel sender.

Describe the solution you'd like
A UnboundedSender::len() method which returns how many messages are buffered in the channel and pending to be received.

// Currently possible
let (boundex_tx, bounded_rx) = mpsc::channel(10);
let bounded_len = bounded_tx.max_capacity() - bounded.capacity();

// Proposed `len` method
let (unbounded_tx, unbounded_rx) = mpsc::unbounded_channel();
let unbounded_len = unbounded_tx.len();

[conradludgate]

For unbounded channels, the semaphore used counts up by 2 for each message sent, and subtracts 2 for each message received. So half the semaphore count could be used for such an implementation.

I do wonder what the usecase is for knowing the length from the sender. There's not a lot you can safely do with it (while avoiding race conditions)

[tqwewe]

Another related issue is #6314.

I do wonder what the usecase is for knowing the length from the sender.

In my case, I'm implementing a least-loaded kind of scheduler for a pool of actors. The pool stores the sender of each actor, and in order to forward a message to the actor with the least amount of load I could pick the actor in the pool with the smallest .len() value.

The alternative of course would be that the actor pool keeps a counter for each actor, but this brings some restrictions, particularly when the worker actors are generic actors defined by the user.