util/usermetrics: make usermetrics non-global

this commit changes usermetrics to be non-global, this is a building
block for correct metrics if a go process runs multiple tsnets or
in tests.

Updates #13420
Updates tailscale/corp#22075

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

wgengine/userspace_ext_test.go

@@ -22,6 +22,7 @@ func TestIsNetstack(t *testing.T) {
 		wgengine.Config{
 			SetSubsystem:  sys.Set,
 			HealthTracker: sys.HealthTracker(),
+			Metrics:       sys.UserMetricsRegistry(),
 		},
 	)
 	if err != nil {
@@ -72,6 +73,7 @@ func TestIsNetstackRouter(t *testing.T) {
 			conf := tt.conf
 			conf.SetSubsystem = sys.Set
 			conf.HealthTracker = sys.HealthTracker()
+			conf.Metrics = sys.UserMetricsRegistry()
 			e, err := wgengine.NewUserspaceEngine(logger.Discard, conf)
 			if err != nil {
 				t.Fatal(err)