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>

llama_stack/apis/providers/providers.py

@@ -6,7 +6,7 @@
 
 from typing import Any, Protocol, runtime_checkable
 
-from pydantic import BaseModel
+from pydantic import BaseModel, HttpUrl, field_validator
 
 from llama_stack.providers.datatypes import HealthResponse
 from llama_stack.schema_utils import json_schema_type, webmethod
@@ -19,6 +19,16 @@ class ProviderInfo(BaseModel):
     provider_type: str
     config: dict[str, Any]
     health: HealthResponse
+    metrics: str | None = None  # define as string type than httpurl for openapi compatibility
+
+    @field_validator("metrics")
+    @classmethod
+    def validate_metrics_url(cls, v):
+        if v is None:
+            return None
+        if isinstance(v, str):
+            HttpUrl(v)
+            return v
 
 
 class ListProvidersResponse(BaseModel):