add prometheus metric

[gangli113]

Fix #19324

Add a prometheus metric etcd_server_feature_enabled

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

Hi @gangli113. 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.

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[gangli113]

/cc @ahrtr

[gangli113]

sorry @ahrtr, assign to you by mistake. I was trying to add you to reviewer list.

[gangli113]

/cc @ahrtr

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: gangli113
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.02%. Comparing base (deb9178) to head (1ca78b3).
Report is 19 commits behind head on main.

❗ Current head 1ca78b3 differs from pull request most recent head ebf03a7

Please upload reports for the commit ebf03a7 to get more accurate results.

Additional details and impacted files

Files with missing lines	Coverage Δ
pkg/featuregate/feature_gate.go	89.54% <100.00%> (+1.18%)	⬆️
server/etcdserver/metrics.go	80.85% <100.00%> (+0.41%)	⬆️
server/etcdserver/server.go	82.67% <100.00%> (+0.90%)	⬆️

... and 26 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19452      +/-   ##
==========================================
+ Coverage   68.90%   69.02%   +0.12%     
==========================================
  Files         420      420              
  Lines       35753    35771      +18     
==========================================
+ Hits        24636    24692      +56     
+ Misses       9693     9660      -33     
+ Partials     1424     1419       -5     

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