Use Tracer instead of LegacyTracer

[natikgadzhi]

Now that we have Swift 5.7 minimum, we can use the new Tracer — not that we have to, LegacyTracer not being deprecated yet.

I don't think we're going to get a lot from having a concrete Span type either, in our case — if I'm reading it right, then we're setting the metadata here in TracingMiddleware.

Could be a good first issue for someone just playing around with Hummingbird and tracing?

[adam-fowler]

Yeah I noticed we could update to using Tracer now. I hadn't got around to doing anything about it. Thank you for the issue. I'm happy to accept PRs as well to resolve this.

[natikgadzhi]

I’m just learning the whole tracing layer of things, but I would love to work on this! Expect a PR in the next couple of days. Thank you for being open to it ❤️