Introduce Prometheus metrics

[hackerwins]

Yorkie is getting more complex as features are added. So, we need to monitor the status of Agent.
It would be nice to support Prometheus /metrics and monitor them in yorkie.dev.

• ETCD: https://github.com/etcd-io/etcd/blob/master/etcdserver/metrics.go

[daclouds]

Could I try this issue?

before I start, I have a question.
Is there any custom application-specific metrics we need?
(In the example of etcd, custom metrics related to leader are registered.)

[hackerwins]

@daclouds Sure. Thank you for your interest in this issue.

I think We need to record snapshot related metrics.

• Yorkie: https://github.com/yorkie-team/yorkie/blob/master/yorkie/packs/pack_service.go#L119
• ETCD: etcd-io/etcd@6764509#diff-4e0502c268b93a783e61e10d1ac1cb99R104-R108

Also, I think It would be nice to save the basics at the beginning of the work(e.g. currentVersion, currentGoVersion).