fix(metric): avoid metrics mux conflict across instances

metric/metric.go

@@ -103,10 +103,11 @@ func (m *metric) Run(ctx context.Context) (err error) {
 		return
 	}
 	if m.config.Addr != "" {
-		var errCh = make(chan error)
-		http.Handle("/metrics", promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{}))
+		var errCh = make(chan error, 1)
+		mux := http.NewServeMux()
+		mux.Handle("/metrics", promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{}))
 		go func() {
-			errCh <- http.ListenAndServe(m.config.Addr, nil)
+			errCh <- http.ListenAndServe(m.config.Addr, mux)
 		}()
 		select {
 		case err = <-errCh:

metric/metric_run_test.go

@@ -0,0 +1,37 @@
+package metric
+
+import (
+	"context"
+	"testing"
+
+	"github.com/anyproto/any-sync/app"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func TestRun_DoesNotPanicWithMultipleInstances(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("metric.Run should not panic with multiple metric instances: %v", r)
+		}
+	}()
+
+	first := newTestMetric()
+	if err := first.Run(context.Background()); err == nil {
+		t.Fatal("first metric run should return listen error with invalid test address")
+	}
+
+	second := newTestMetric()
+	if err := second.Run(context.Background()); err == nil {
+		t.Fatal("second metric run should return listen error with invalid test address")
+	}
+}
+
+func newTestMetric() *metric {
+	return &metric{
+		registry: prometheus.NewRegistry(),
+		// Intentionally invalid address to avoid creating long-lived listeners in unit tests.
+		config:      Config{Addr: "127.0.0.1"},
+		a:           &app.App{},
+		syncMetrics: map[string]SyncMetric{},
+	}
+}