[Bugfix][Rust Frontend] Make metrics respect --served-model-name (#45465)

Signed-off-by: reidliu41 <reid201711@gmail.com>

rust/src/server/src/lib.rs

@@ -35,9 +35,24 @@ use crate::routes::build_router;
 use crate::server_info::ServerInfoSnapshot;
 use crate::state::AppState;
 
+/// Resolve the public model names accepted by the frontend.
+fn effective_served_model_names(model: &str, served_model_name: &[String]) -> Vec<String> {
+    if served_model_name.is_empty() {
+        vec![model.to_string()]
+    } else {
+        served_model_name.to_vec()
+    }
+}
+
 /// Build the shared application state for one configured model and one engine
 /// client.
 async fn build_state(config: &Config) -> Result<Arc<AppState>> {
+    // If no served names are specified, fall back to the backend model path so
+    // that the API always has at least one valid model ID. Use the same primary
+    // public name for frontend-side metrics labels.
+    let served_model_names = effective_served_model_names(&config.model, &config.served_model_name);
+    let metrics_model_name = served_model_names[0].clone();
+
     // Load both backends from the same model metadata so they stay in sync.
     let loaded = load_model_backends(
         &config.model,
@@ -68,7 +83,7 @@ async fn build_state(config: &Config) -> Result<Arc<AppState>> {
     let client = EngineCoreClient::connect(EngineCoreClientConfig {
         transport_mode: config.transport_mode.clone(),
         coordinator_mode,
-        model_name: config.model.clone(),
+        model_name: metrics_model_name,
         client_index: 0,
     })
     .await
@@ -81,14 +96,6 @@ async fn build_state(config: &Config) -> Result<Arc<AppState>> {
         .with_tool_call_parser(config.tool_call_parser.clone())
         .with_reasoning_parser(config.reasoning_parser.clone());
 
-    // If no served names are specified, fall back to the backend model path so
-    // that the API always has at least one valid model ID.
-    let served_model_names = if config.served_model_name.is_empty() {
-        vec![config.model.clone()]
-    } else {
-        config.served_model_name.clone()
-    };
-
     Ok(Arc::new(
         AppState::new(served_model_names, chat)
             .with_api_server_options(config.api_server_options)
@@ -258,3 +265,26 @@ where
         .unwrap_or_else(|| Instant::now() + config.shutdown_timeout);
     state.shutdown(shutdown_deadline).await
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn effective_served_model_names_falls_back_to_backend_model() {
+        assert_eq!(
+            effective_served_model_names("backend-model", &[]),
+            vec!["backend-model"]
+        );
+    }
+
+    #[test]
+    fn effective_served_model_names_preserves_public_names() {
+        let served_names = vec!["public-model".to_string(), "public-alias".to_string()];
+
+        assert_eq!(
+            effective_served_model_names("backend-model", &served_names),
+            served_names
+        );
+    }
+}

rust/src/server/src/routes/tests.rs

@@ -1677,6 +1677,85 @@ async fn http_metrics_record_list_models_requests() {
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+#[serial]
+async fn request_metrics_use_served_model_name_label() {
+    let ipc = IpcNamespace::new().expect("create ipc namespace");
+    let handshake_address = ipc.handshake_endpoint();
+    let engine_id = b"engine-openai-served-model-metrics".to_vec();
+
+    let engine_task = MockEngineTask::new(spawn_mock_engine_task(
+        handshake_address.clone(),
+        engine_id.clone(),
+        |dealer, push| {
+            boxed_test_future(async move {
+                let add = recv_engine_message(dealer).await;
+                let request: EngineCoreRequest =
+                    rmp_serde::from_slice(&add[1]).expect("decode request");
+                send_outputs(
+                    push,
+                    engine_outputs_for_request(&request.request_id, default_stream_output_specs()),
+                )
+                .await;
+            })
+        },
+    ));
+
+    let client = EngineCoreClient::connect(
+        EngineCoreClientConfig::new_single(handshake_address)
+            .with_model_name("served-model-metrics")
+            .with_local_input_output_addresses(
+                Some(ipc.input_endpoint()),
+                Some(ipc.output_endpoint()),
+            ),
+    )
+    .await
+    .expect("connect client");
+    let chat = ChatLlm::from_shared_backend(test_llm(client), Arc::new(FakeChatBackend::new()));
+    let mut app = build_router(Arc::new(AppState::new(
+        vec![
+            "served-model-metrics".to_string(),
+            "served-model-alias".to_string(),
+        ],
+        chat,
+    )));
+    let before = METRICS.render().unwrap();
+
+    let response = app
+        .call(
+            Request::builder()
+                .method("POST")
+                .uri("/v1/chat/completions")
+                .header("content-type", "application/json")
+                .body(Body::from(
+                    json!({
+                        "model": "served-model-alias",
+                        "stream": false,
+                        "messages": [{"role": "user", "content": "hello"}]
+                    })
+                    .to_string(),
+                ))
+                .expect("build request"),
+        )
+        .await
+        .expect("call app");
+
+    assert_eq!(response.status(), StatusCode::OK);
+    let _ = to_bytes(response.into_body(), usize::MAX).await.unwrap();
+
+    let after = METRICS.render().unwrap();
+    assert_eq!(
+        metric_delta(
+            &before,
+            &after,
+            "vllm:request_success_total",
+            Some("model_name=\"served-model-metrics\",engine=\"0\",finished_reason=\"stop\""),
+        ),
+        1.0
+    );
+    engine_task.await.expect("mock engine task");
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 #[serial]
 async fn wrong_model_returns_not_found() {