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>

tests/integration/providers/test_providers.py

@@ -19,3 +19,13 @@ class TestProviders:
             pid = provider.provider_id
             provider = llama_stack_client.providers.retrieve(pid)
             assert provider is not None
+
+    @pytest.mark.asyncio
+    def test_providers_metrics_field(self, llama_stack_client: LlamaStackAsLibraryClient | LlamaStackClient):
+        """Test metrics field is in provider responses."""
+        provider_list = llama_stack_client.providers.list()
+        assert provider_list is not None
+        assert len(provider_list) > 0
+
+        for provider in provider_list:
+            assert provider.metrics is None or isinstance(provider.metrics, str)