[release-3.6] Add prometheus metric for server feature gate

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #19452

/assign ahrtr

[k8s-ci-robot]

Hi @k8s-infra-cherrypick-robot. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ahrtr]

/ok-to-test

[ahrtr]

cc @fuweid @ivanvc @siyuanfoundation

[ahrtr]

Link to #19489

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.91%. Comparing base (12ac41a) to head (27e3863).
Report is 2 commits behind head on release-3.6.

Additional details and impacted files

Files with missing lines	Coverage Δ
pkg/featuregate/feature_gate.go	88.96% <ø> (+0.60%)	⬆️
server/etcdserver/metrics.go	80.85% <100.00%> (+0.41%)	⬆️
server/etcdserver/server.go	82.26% <100.00%> (-0.41%)	⬇️

... and 23 files with indirect coverage changes

@@               Coverage Diff               @@
##           release-3.6   #19495      +/-   ##
===============================================
- Coverage        68.93%   68.91%   -0.03%     
===============================================
  Files              420      420              
  Lines            35779    35788       +9     
===============================================
- Hits             24666    24662       -4     
- Misses            9688     9705      +17     
+ Partials          1425     1421       -4     

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 12ac41a...27e3863. Read the comment docs.

[ivanvc]

Backport LGTM on green.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, ivanvc, k8s-infra-cherrypick-robot

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [ahrtr]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment