feat(BA-356): Add prometheus metrics

[HyeockJinKim]

resolves https://github.com/lablup/backend.ai-internal/issues/93 (BA-356)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

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📚 Documentation preview 📚: https://sorna--3457.org.readthedocs.build/en/3457/

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📚 Documentation preview 📚: https://sorna-ko--3457.org.readthedocs.build/ko/3457/