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>

ipn/ipnlocal/serve_test.go

@@ -684,6 +684,7 @@ func newTestBackend(t *testing.T) *LocalBackend {
 	e, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
 		SetSubsystem:  sys.Set,
 		HealthTracker: sys.HealthTracker(),
+		Metrics:       sys.UserMetricsRegistry(),
 	})
 	if err != nil {
 		t.Fatal(err)