Expose bbr pacing_rate to users (incl. ffi)

[mpelletan]

Hello,

I'm using the library for a prototype but would like my application to limit how much data it feeds into the library for sending, in such a way that congestion (RTT increase, loss) is avoided, without incurring buffering into quiche's send buffer.

To do so I would need to be able to query the current pacing rate from the congestion control, but that's not currently exposed. I have a version of this change locally to unblock myself but proper support / consideration of any congestion control algorithm would of course be preferred. My version is a simple forwarding of the bbr2 state's pacing rate through path stats (and "null" if not using bbr).

Thanks!

[LPardue]

This seems like something reasonable to expose in PathStats, we already record it in qlog but that's too late /awkward to be useful for anyone.