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>

cmd/tailscaled/tailscaled.go

@@ -680,12 +680,15 @@ func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack boo
 		ListenPort:    args.port,
 		NetMon:        sys.NetMon.Get(),
 		HealthTracker: sys.HealthTracker(),
+		Metrics:       sys.UserMetricsRegistry(),
 		Dialer:        sys.Dialer.Get(),
 		SetSubsystem:  sys.Set,
 		ControlKnobs:  sys.ControlKnobs(),
 		DriveForLocal: driveimpl.NewFileSystemForLocal(logf),
 	}
 
+	sys.HealthTracker().SetMetricsRegistry(sys.UserMetricsRegistry())
+
 	onlyNetstack = name == "userspace-networking"
 	netstackSubnetRouter := onlyNetstack // but mutated later on some platforms
 	netns.SetEnabled(!onlyNetstack)