Add orphan GC sweeper and Prometheus metrics with explicit registration

Co-Authored-By: Claude <noreply@anthropic.com>

internal/discovery/handlers.go

@@ -172,6 +172,9 @@ func (s *Server) handleAcquireLease(w http.ResponseWriter, r *http.Request) {
 		TTL:        ttl,
 	})
 	if errors.Is(err, lease.ErrNoMatch) {
+		if s.Metrics != nil {
+			s.Metrics.LeaseRequest("no_match")
+		}
 		writeJSON(w, http.StatusConflict, map[string]any{
 			"error":      "no_match",
 			"message":    "no healthy proxy with free capacity matched the selector",
@@ -187,6 +190,9 @@ func (s *Server) handleAcquireLease(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if s.Metrics != nil {
+		s.Metrics.LeaseRequest("granted")
+	}
 	writeJSON(w, http.StatusCreated, leaseResponse{
 		LeaseID:    granted.ID,
 		Proxy:      viewOf(byKey[granted.Proxy], s.Store.Counts()[granted.Proxy]),