proposal: support instrumentation

[groob]

It would be useful if the cert manager had support for metrics.
Some useful data to keep track of:

• number of certs issued/renewed
• time to acquire a cert
• number of errors

I use prometheus to monitor our k8s services, but some people use other monitoring solutions. I'd like to add metrics support to this projects, using the github.com/go-kit/kit/metrics library with initial support for prometheus and expvar endpoints.

Would you be interested in this? Any thoughts/concerns?

[luna-duclos]

As long as things keep working if there is no metrics endpoint available, yes, this would be absolutely very interesting. It could also be mixed with #13 and #12 as it'd probably touch the same code