feat: signer monitoring server

[hstove]

• Closes Nakamoto Signer[3.0] - Expose prometheus metrics from stacks-signer #4707

This adds an (optional) metrics/monitoring server the the stacks-signer. It's designed similarly to the node's metrics server, where the server runs optionally and behind a monitoring_prom feature flag.

There is a new signer config option, metrics_endpoint, which specifies where the server should listen.

There are a bunch of metrics that are relevant to signer operation, but I might have missed some things that would be good to add! Please let me know if you think a metric is missing or should be changed.

The available routes exposed are:

• /metrics: prometheus metrics
• /info: JSON about the currently running signer
• /heartbeat: Check the connection from the signer to the node. Returns 200 or 500.
• /: 200 ok

With these routes, my hope is that this can help with signers answering the "is this thing running correctly" question.

[hstove]

One thought is that it'd be good to have some timing metrics here, like roundtrip time for signer -> node RPC calls.

EDIT: added ✅

[kantai]

This looks good to me, just had a few comments.

[obycode]

Looks good. No additional comments outside of those Aaron already covered. Will approve once those are addressed. 👍

[obycode]

Thanks!