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>

tsnet/tsnet.go

@@ -536,12 +536,14 @@ func (s *Server) start() (reterr error) {
 		SetSubsystem:  sys.Set,
 		ControlKnobs:  sys.ControlKnobs(),
 		HealthTracker: sys.HealthTracker(),
+		Metrics:       sys.UserMetricsRegistry(),
 	})
 	if err != nil {
 		return err
 	}
 	closePool.add(s.dialer)
 	sys.Set(eng)
+	sys.HealthTracker().SetMetricsRegistry(sys.UserMetricsRegistry())
 
 	// TODO(oxtoacart): do we need to support Taildrive on tsnet, and if so, how?
 	ns, err := netstack.Create(tsLogf, sys.Tun.Get(), eng, sys.MagicSock.Get(), s.dialer, sys.DNSManager.Get(), sys.ProxyMapper(), nil)