Prometheus exporter

[soleinik-figment]

In brief

This change adds Prometheus metrics exports for observability. Expose session stats counters, such as PreCommits,Proposals, PreVotes in addition to errors StateErrors (all inclusive errors, including DoubleSign ) and DoubleSign (by itself).

What's the usecase?

To pause signing in double signing possibility, to avoid slashing

[tony-iqlusion]

As a general comment, this seems to add quite a bit of code.

I've been looking at https://autometrics.dev/ as a potential alternative. It uses a proc macro-based approach which makes it easy to instrument any function:

https://github.com/autometrics-dev/autometrics-rs/

(It still uses prometheus-client under the hood)

[tony-iqlusion]

Closing as stale.

It'd be great to have Prometheus support, but IMO this PR is way too low-level and includes a lot of code which https://autometrics.dev/ would abstract away.

[DavidVentura]

Hey @tony-iqlusion, I want to make a PR using autometrics, however, the crate is no longer maintained.

Do you have an opinion on whether that's fine, or you'd rather use something else?

Alternatively, I'm fairly sure I can make a PR using prometheus with a minimal footprint.