Expose a function for retrieving stats about a connection

[andrewvy]

I wanted a way for exposing ConnectionStats and came up with a quick POC of storing the internal Quinn connection when the successful connection is made. This lets you call .stats() on it later on, which is useful for grabbing the RTT / net stats for observability.

I'm not sure if this is the best way about going about it, so I'm up for amending this in whichever way you see fit! (I'm not sure if we want to just re-export the ConnectionStats struct from quinn-proto, it seems quinn itself doesn't re-export this for some reason.. )

[Henauxg]

Thank you for this PR !

I took the liberty to make a few changes :

• Store the internal Quinn connection inside the connection's Connected state
• I finally went with the re-export of quinn-proto ConnectionStats. I did hesitate a lot and still am not 100% sure about this too. A few reasons that made me decide:
  • this feels simpler than maintaining a duplicate of quinn-proto stats structs
  • for now, Quinn is the only back-end, so exposing some of it is not a problem
  • this also has the benefit that all the existing information are available right away, we won't have to add more and more to fulfill possible future needs
  • this is not a critical part of the API, we can still change it later if need be

If this all seems good to you, I can go ahead and merge it

[andrewvy]

@Henauxg Yep, works for me!