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.html

@@ -11658,6 +11658,9 @@
                                 }
                             ]
                         }
+                    },
+                    "metrics": {
+                        "type": "string"
                     }
                 },
                 "additionalProperties": false,