chore(metrics): github page doc

[flochaz]

Description of your changes

Add doc for metrics module

How to verify this change

1. build the doc with npm run docs-runLocalDocker
2. go to http://localhost:8000/core/metrics/

Related issues, RFCs

#151

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

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[dreamorosi]

Looking great! No issue with the content, just some minor formatting details (tried to provide suggestions for most of them).