Expose WebRTC peerconn stats

[marcovidonis]

I revisited the work on #803 and improved on it. By saving a pointer to the open peer connections, we get stats more reliably. Stats now also work when seeding.

A note on the Pion/webrtc version. While some stats are available in the current latest version (v3), the project greatly improved on them in v4, currently in beta. I'd recommend upgrading to v4 as soon as it's convenient when it gets out of beta.

[anacrolix]

I'm happy to delegate this to you entirely. I don't imagine it's easy to add some tests here? It's probably too much work to spin up a fake websocket server, but that's about my only suggestion.

Let me know when you're happy with the state of the PR and I'll merge it (including now if you think it's ready now).

I also don't really know much and am not particularly familiar with WebRTC, so if you are interested in a fork or long-lived branch to test using v4 of pion, I'm open to that.

Thanks!

[marcovidonis]

That's great, thanks! Yeah I don't really see how to make unit tests for this... we'd likely need an integration test of some kind.

I think the PR is almost ready now, as it does expose the stats we get from Pion. I noticed the wasm test fails because Pion doesn't yet support stats on wasm. Do you know of a way I could avoid calling GetStats() if we're on a wasm architecture?

[anacrolix]

I'd probably put the GetStats behind a build tag that matches whatever pion uses. For the unsupported tags, return something appropriate.

[marcovidonis]

Thanks for that, @anacrolix! All checks pass now, so I think you can merge if you're happy with this.

Also, I think a long-lived branch for Pion v4 could be useful, so I'd be happy to make one!

[anacrolix]

Go for it, thanks!