fix: /v2/stackerdb metrics identifier typo

[Jiloc]

Description

I initially had this fix in #6201 but decided to move it a separate PR just in case.

Applicable issues

• fixes #

Additional info (benefits, drawbacks, caveats)

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.95%. Comparing base (6168284) to head (b979ca6).
⚠️ Report is 65 commits behind head on develop.

❌ Your project status has failed because the head coverage (75.95%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (6168284) and HEAD (b979ca6). Click for more details.

HEAD has 14 uploads less than BASE

Flag	BASE (6168284)	HEAD (b979ca6)
	133	119

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6296      +/-   ##
===========================================
- Coverage    81.45%   75.95%   -5.50%     
===========================================
  Files          540      540              
  Lines       347695   347695              
===========================================
- Hits        283225   264108   -19117     
- Misses       64470    83587   +19117     

Files with missing lines	Coverage Δ
stackslib/src/net/api/liststackerdbreplicas.rs	90.00% <100.00%> (ø)

... and 225 files with indirect coverage changes

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[github-actions]

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