feat: add optional metrics under API /providers

- add field "metrcis" under API "/providers"
- each provider can config "metrics" in run.yaml in "config"
  - if no "metrics":  /provider/<provider_id> shows "metrics: null" in response
  - if has "metrics": /provider/<provider_id> show result in response
  - if has "metrics" but is not string type and not httpurl, raise ValidationError
- add unit tests for providers
- update "docs"

Signed-off-by: Wen Zhou <wenzhou@redhat.com>

docs/_static/llama-stack-spec.yaml

@@ -8215,6 +8215,8 @@ components:
               - type: string
               - type: array
               - type: object
+        metrics:
+          type: string
       additionalProperties: false
       required:
         - api