add socket-load-balance flag

[graingert]

Summary

Fixes #2467

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[graingert]

Needs python/cpython#124761

[Kludex]

Why do we need to add a new argument i.e. socket-load-balance? Can't this behavior be the default? I'd assume people already thinks that's the default.

[graingert]

There's a tradeoff where the default unfair load balancing favours lower latency and the SO_REUSEPORT(_LB) approach favours an even load between processes

I suspect most people do want the fair load balancing, however I think it should bed in a bit before we suddenly turn it on by default for everyone.

[graingert]

this needs a test for two independent calls to Multiprocess(...).run() running on the same port:

def test_bind_to_used_port():
    port = ephemeral_port_reserve.reserve() 
    config = Config(app=app, workers=2, socket_load_balance=True, port=port, interface="asgi3")
    with multiprocessing.get_context("spawn").Pool(max_workers=1) as pool:
        try:
            f1 = pool.apply_async(uvicorn.main, ...)
            # wait for started
            with pytest.raises(OSError, match="already bound"):
                uvicorn.main(...)
        finally: pool.terminate()

[vvanglro]

Wouldn't it be better to put the logic here in config.bind_socket?
Just add it to the else, uds and fd don't seem to support SO_REUSEPORT.

[graingert]

hmm I'm not sure about this, I want to make sure the SO_REUSEPORT flag isn't set before the socket is bound so we still fail if another uvicorn is using this port

[vvanglro]

How about this?
vvanglro@f2b1037