core/fetcher: add core_fetcher_proposal_blinded metric

[DiogoSantoss]

Add core_fetcher_proposal_blinded metric which tracks whether the fetched proposal data is blinded (built by MEV) or locally built.

category: feature
ticket: #4106

[Copilot]

Pull Request Overview

This PR adds a new Prometheus gauge metric core_fetcher_proposal_blinded to track whether fetched beacon block proposals are blinded (built by MEV builders) or locally built by the beacon node. The metric uses a simple binary value where 1 indicates a blinded proposal and 0 indicates a local proposal.

Key Changes:

• Added new metric definition in core/fetcher/metrics.go
• Instrumented the proposal fetching logic to set the metric based on the proposal.Blinded field
• Updated metrics documentation to reflect the new metric

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/metrics.md	Added documentation entry for the new core_fetcher_proposal_blinded gauge metric
core/fetcher/metrics.go	New file defining the proposalBlindedGauge metric using promauto
core/fetcher/fetcher.go	Added instrumentation to set the blinded metric value after fetching each proposal

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.39%. Comparing base (1101cf1) to head (361797d).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4117      +/-   ##
==========================================
- Coverage   56.39%   56.39%   -0.01%     
==========================================
  Files         245      245              
  Lines       31169    31173       +4     
==========================================
+ Hits        17578    17580       +2     
  Misses      11284    11284              
- Partials     2307     2309       +2     

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