feat: show inference metrics from Endpoint and Routing queries

[kyujin-cho]

This PR brings following updates:

• Huggingface TGI runtime variant support
• Show inference metric from GQL query
  • This feature will only be effective when using Enterprise AppProxy, as we do not have plans to support such feature on OSS AppProxy

Inference metric format

{
  # ...
  "vllm_request_params_n": {
    "__type": "HISTOGRAM",
    "current": {
      "1.0": "3832319",
      "2.0": "3832319",
      "5.0": "3832319",
      "10.0": "3832319",
      "20.0": "3832319",
      "+Inf": "3832319"
    },
    "threshold_unit": "le",
    "count": 3832319,
    "sum": "3832319"
  },
  "vllm_request_params_best_of": {
    "__type": "HISTOGRAM",
    "current": {
      "1.0": "3832319",
      "2.0": "3832319",
      "5.0": "3832319",
      "10.0": "3832319",
      "20.0": "3832319",
      "+Inf": "3832319"
    },
    "threshold_unit": "le",
    "count": 3832319,
    "sum": "3832319"
  },
  "vllm_cache_config_info": {
    "__type": "GAUGE",
    "current": "1",
    "capacity": null,
    "pct": "0.00",
    "unit_hint": "count"
  },
  # ...
}

Metrics from inference frameworks can be distinguished by three types: GAUGE, COUNTER and HISTOGRAM. The identifier __type bundled at the root of every metrics represents its type. GAUGE, COUNTER follows Backend.AI's session metrics format, whereas HISTOGRAM has its own new defintion.

HISTOGRAM metric format

On HISTOGRAM type, the distribution will be illustrated under current key. count and sum are optional, and will be supplied only if inference framework supports.

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