mvcc: add store revision metrics

[jingyih]

Add experimental metrics etcd_debugging_mvcc_current_revision and etcd_debugging_mvcc_compact_revision.

[jingyih]

Have we seen this functional test error before?

{"level":"warn","ts":"2019-09-05T22:55:57.643Z","caller":"etcdmain/etcd.go:176","msg":"failed to start etcd","error":"cannot fetch cluster info from peer urls: could not retrieve cluster information from the given URLs"}
{"level":"fatal","ts":"2019-09-05T22:55:57.643Z","caller":"etcdmain/etcd.go:271","msg":"discovery failed","error":"cannot fetch cluster info from peer urls: could not retrieve cluster information from the given URLs","stacktrace":"go.etcd.io/etcd/etcdmain.startEtcdOrProxyV2\n\t/go/src/go.etcd.io/etcd/etcdmain/etcd.go:271\ngo.etcd.io/etcd/etcdmain.Main\n\t/go/src/go.etcd.io/etcd/etcdmain/main.go:46\nmain.main\n\t/go/src/go.etcd.io/etcd/main.go:28\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:200"}

[jingyih]

cc @wenjiaswe

[wenjiaswe]

@jingyih No, all functional test passes when running locally.

[wenjiaswe]

lgtm, thanks!

[spzala]

LGTM too. I believe this should be added in the changelog though. Thanks @jingyih !!

[jingyih]

LGTM too. I believe this should be added in the changelog though. Thanks @jingyih !!

Thanks for reminding!

[wenjiaswe]

By the way, I think we could cherry pick metrics back to previous versions right? Could we cherry pick it to 3.2, 3.3. and 3.4?

[jingyih]

By the way, I think we could cherry pick metrics back to previous versions right? Could we cherry pick it to 3.2, 3.3. and 3.4?

Sounds good.