Expose app_limited and send_rate in recovery events

[zz85]

Problem:

Would be great for an application to know

a) when s2n-quic is app_limited

this could be similar to the congestion_limited event that is emitted.

b) send/delivery rate

While delivery rate could be calculated on packet sent and ack received events, it'll be additional work to compute at application layer as it requires packet tracking that's already done at the transport layer.

Solution:

One point of reference is the TCP delivery rate implemented in the Linux kernel.

• https://datatracker.ietf.org/doc/html/draft-cheng-iccrg-delivery-rate-estimation
• https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/tcp_rate.c

Requirements / Acceptance Criteria:

Out of scope: