Add prometheus metrics

[dmitryilyin]

Add Prometheus metrics endpoint at "/metrics" with topic and consumer group measurements.

[bai]

Thanks for contributing. Could you please add tests for it?

[coveralls]

Coverage decreased (-3.7%) to 71.012% when pulling 272f470 on dmitryilyin:prometheus_metrics into a449cc4 on linkedin:master.

[dmitryilyin]

Hello! Ok, I'll make tests.

[mrueg]

See also: https://github.com/jirwin/burrow_exporter/ (it might make sense to make use of prometheus golang libraries in your PR)

[shamil]

Related issue #318

[JensRantil]

Have you considered using https://godoc.org/github.com/prometheus/client_golang/prometheus instead of generating the https://prometheus.io/docs/instrumenting/exposition_formats/ manually yourself? The benefit of using the library is that you likely won't need to make any code changes if the exposition format changes. Additionally, you don't need to take care of string escaping and other low-level things.

[ledor473]

Any update on this pull request?

@bai is there a chance to get that merged if tests are added?
Is there even better chance it's merged if the client_golang is used?

@dmitryilyin do you have the bandwidth to tackle this or would you need help?

[bai]

I'd personally prefer having a native client_golang implementation since it's proven in production. I personally have no bandwidth to implement it though.

I won't hesitate reviewing a PR and releasing it if you or someone else would implement it 🙏

[paoloa21]

When will the feature be released?
Where Can I find the documentation for prometheus exporter?