quinn-proto: `ConnectionStats` field types are not exported

[abonander]

UdpStats, FrameStats and PathStats are not exported so their docs are not rendered:

The fields of these types are public and have doc-comments, suggesting this is just an oversight: https://github.com/quinn-rs/quinn/blob/3eb26361dba85f13b69e0eff6d934b28f70a37f8/quinn-proto/src/connection/stats.rs

[Ralith]

Yep, those should be exported; good catch.