feat: Add prometheus client

[HyeockJinKim]

I added prometheus metrics for api & events.
It seems that metrics for Kernel, Agent, Session, etc. should be collected in Agent, not in Manager.
(It is difficult to collect metrics with the event structure).

Since the event dispatcher exists in common, the event dispatcher only specifies the required Protocol.
When using the event dispatcher in managers and agents, it is intended to work by passing the appropriate metric class.

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--3184.org.readthedocs.build/en/3184/

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📚 Documentation preview 📚: https://sorna-ko--3184.org.readthedocs.build/ko/3184/