lowercase value of metric enable when migrate from old to new version

[thuvh]

Thank you for contributing to Harbor!

when migrate from old to new version with enabled metric, harbor.yml is generated like

metric:
  enabled: True

Issue being fixed

n/a

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.84%. Comparing base (c8c11b4) to head (832b3cd).
⚠️ Report is 595 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #22580       +/-   ##
===========================================
+ Coverage   45.36%   65.84%   +20.47%     
===========================================
  Files         244     1073      +829     
  Lines       13333   116095   +102762     
  Branches     2719     2931      +212     
===========================================
+ Hits         6049    76446    +70397     
- Misses       6983    35409    +28426     
- Partials      301     4240     +3939     

Flag	Coverage Δ
unittests	65.84% <ø> (+20.47%)	⬆️

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see 987 files with indirect coverage changes

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[github-actions]

This PR is being marked stale due to a period of inactivty. If this PR is still relevant, please comment or remove the stale label. Otherwise, this PR will close in 30 days.

[thuvh]

@MinerYang @stonezdj @Vad1mo please review for me