Exposing prometheus/opentelemetry compatible metrics.

[jcgruenhage]

Related to #80 and kinda also #24, it'd be helpful to have better insight into what acmed is doing, by exposing certain metrics on it. As everything in acmed is related to http requests in one form or another, it's probably enough to expose counters for http requests, including labels for

• endpoint
• acme resource
• path <- this one should probably be optional, due to it blowing up the size of the metrics collected when there is a lot of certificates
• status code

In addition to that, it might also make sense to export some metrics on a few other areas, like the hooks and rate limit state, but I think those metrics are less useful than the ones for the http requests.

While it's also possible to expose certain metrics for the certificates themselves, I'm not sure whether that makes sense, as it's easily possible to do externally (see https://gist.github.com/jcgruenhage/8ef2f2fbc0af48189ee4585fb8054e40 for example), where as these are metrics only available to acmed itself.