22786 expose registry health as prometheus metrics

[cr3ativ3cod3r]

Thank you for contributing to Harbor!

Comprehensive Summary of your change

This PR adds a Prometheus metric to expose remote registry health status.

• Adds a new collector 'harbor_registry_status'
• The collector iterates through the registries to get their health status
• Exposes their health in the /metrics endpoint

Issue being fixed

Fixes #22786

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

❌ Patch coverage is 57.50000% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.92%. Comparing base (c8c11b4) to head (edcbc77).
⚠️ Report is 667 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pkg/exporter/registry_collector.go	58.97%	16 Missing ⚠️
src/pkg/exporter/exporter.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #22811       +/-   ##
===========================================
+ Coverage   45.36%   65.92%   +20.55%     
===========================================
  Files         244     1075      +831     
  Lines       13333   116451   +103118     
  Branches     2719     2937      +218     
===========================================
+ Hits         6049    76766    +70717     
- Misses       6983    35438    +28455     
- Partials      301     4247     +3946     

Flag	Coverage Δ
unittests	65.92% <57.50%> (+20.55%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/pkg/exporter/exporter.go	6.55% <0.00%> (ø)
src/pkg/exporter/registry_collector.go	58.97% <58.97%> (ø)

... and 988 files with indirect coverage changes

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