chore(lint): enable promlinter in golangci-lint

[ADITYATIWARI342005]

What type of PR is this?

chore: CI & build tools improvement - enables promlinter to catch common Prometheus client usage issues

What this PR does / why we need it:

This PR enables the promlinter in golangci-lint to catch common Prometheus client usage issues and improve code quality for metrics-related code.

Changes made:

• Linter Configuration: Updates tools/linter/golangci-lint/.golangci.yml to add promlinter with minimal settings (strict: false)

Impact:

• CI will now report Prometheus metrics lint issues in future PRs
• Local linter runs confirmed promlinter is active and integrated
• Subsequent PRs modifying Prometheus metrics may need to address findings

Files Changed:

• tools/linter/golangci-lint/.golangci.yml: Added promlinter configuration

Testing:

• ✅ Local golangci-lint run performed
• ✅ Promlinter confirmed active and integrated
• ✅ No new failures from promlinter on current codebase

Which issue(s) this PR fixes:
#7054

Closes #7054

Release Notes: No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.98%. Comparing base (c4eed01) to head (a8d0620).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7063      +/-   ##
==========================================
- Coverage   71.02%   70.98%   -0.04%     
==========================================
  Files         227      227              
  Lines       40465    40465              
==========================================
- Hits        28741    28726      -15     
- Misses      10021    10034      +13     
- Partials     1703     1705       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[arkodg]

thanks @ADITYATIWARI342005, is there a simple way to prove that this linter is actually useful by for e.g. editing these metric names https://github.com/envoyproxy/gateway/blob/main/internal/message/metrics.go and seeing if the linter catches it ?

[ADITYATIWARI342005]

@arkodg

Yes, the promlinter is proven useful,
I tested it by creating metrics with common violations in a test file:

• counter metrics missing _total suffix
• the linter immediately caught both violations
• this will help in following Prometheus naming conventions automatically

[arkodg]

@ADITYATIWARI342005 can your edit the above suggested file to see if it still performs similarly? We are using wrappers so want to make sure it works

[ADITYATIWARI342005]

Ok I will test the changes and inform you in some time.

[ADITYATIWARI342005]

@arkodg
Testing Results on Real Metrics Files:

• Promlinter is working correctly - I tested it by adding violations to internal/message/metrics.go:
  Found:

• Promlinter catches violations in direct Prometheus client usage (promauto.NewCounter)

• ⚠️ Promlinter does NOT catch violations in the custom wrapper system (metrics.NewCounter)

Promlinter will catch issues if developers use direct Prometheus client calls.

[arkodg]

thanks for testing this out @ADITYATIWARI342005, I dont think there's value in adding this linter then because we've chosen to use internal libs to generate metrics
cc @Xunzhuo

[ADITYATIWARI342005]

@arkodg
Thank you for your review, & We may close issue #7054 then & this PR as well.