[model-gateway] add back router worker health metric and fix init state (#15622)

sgl-model-gateway/src/core/worker.rs

@@ -588,6 +588,7 @@ impl Worker for BasicWorker {
 
     fn set_healthy(&self, healthy: bool) {
         self.healthy.store(healthy, Ordering::Release);
+        Metrics::set_worker_health(self.url(), healthy);
     }
 
     async fn check_health_async(&self) -> WorkerResult<()> {

sgl-model-gateway/src/core/worker_builder.rs

@@ -9,7 +9,7 @@ use super::{
         WorkerType,
     },
 };
-use crate::routers::grpc::client::GrpcClient;
+use crate::{observability::metrics::Metrics, routers::grpc::client::GrpcClient};
 
 /// Builder for creating BasicWorker instances with fluent API
 pub struct BasicWorkerBuilder {
@@ -187,11 +187,14 @@ impl BasicWorkerBuilder {
             None => OnceCell::new(),
         });
 
+        let healthy = true;
+        Metrics::set_worker_health(&self.url, healthy);
+
         BasicWorker {
             metadata,
             load_counter: Arc::new(AtomicUsize::new(0)),
             processed_counter: Arc::new(AtomicUsize::new(0)),
-            healthy: Arc::new(AtomicBool::new(true)),
+            healthy: Arc::new(AtomicBool::new(healthy)),
             consecutive_failures: Arc::new(AtomicUsize::new(0)),
             consecutive_successes: Arc::new(AtomicUsize::new(0)),
             circuit_breaker: CircuitBreaker::with_config_and_label(

sgl-model-gateway/src/observability/metrics.rs

@@ -174,6 +174,10 @@ pub fn init_metrics() {
         "smg_worker_requests_active",
         "Currently running requests per worker"
     );
+    describe_gauge!(
+        "smg_worker_health",
+        "Worker health status (1=healthy, 0=unhealthy)"
+    );
     describe_counter!(
         "smg_worker_health_checks_total",
         "Health check results by worker_type and result"
@@ -807,6 +811,15 @@ impl Metrics {
         .set(count as f64);
     }
 
+    /// Set worker health status
+    pub fn set_worker_health(worker_url: &str, healthy: bool) {
+        gauge!(
+            "smg_worker_health",
+            "worker" => worker_url.to_string()
+        )
+        .set(if healthy { 1.0 } else { 0.0 });
+    }
+
     // ========================================================================
     // Layer 3: Worker resilience metrics (circuit breaker)
     // ========================================================================