[router] add metrics for worker and policy (#8971)

Signed-off-by: Tony Lu <tonyluj@gmail.com>

sgl-router/src/policies/round_robin.rs

@@ -2,6 +2,7 @@
 
 use super::{get_healthy_worker_indices, LoadBalancingPolicy};
 use crate::core::Worker;
+use crate::metrics::RouterMetrics;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 /// Round-robin selection policy
@@ -35,7 +36,10 @@ impl LoadBalancingPolicy for RoundRobinPolicy {
         // Get and increment counter atomically
         let count = self.counter.fetch_add(1, Ordering::Relaxed);
         let selected_idx = count % healthy_indices.len();
+        let worker = workers[healthy_indices[selected_idx]].url();
 
+        RouterMetrics::record_processed_request(worker);
+        RouterMetrics::record_policy_decision(self.name(), worker);
         Some(healthy_indices[selected_idx])
     }