feat/draft: per-proxy metrics

[Frando]

This uses n0-computer/iroh-proxy-utils#14 to update the per-proxy metrics view with actual metrics for this proxy.

The PR in its current state is a most-minimal implementation that tries to change the existing code as little as possible, to showcase the new metrics support in iroh-proxy-utils. The state handling and metrics sampling can very likely be improved.

Feel free to take over this PR from here!

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ mattdjenkinson
❌ Frando
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

❌ mattdjenkinson
❌ Frando
_{You have signed the CLA already but the status is still pending? Let us recheck it.}