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/magicsock/magicsock.go

@@ -60,6 +60,7 @@ import (
 	"tailscale.com/util/set"
 	"tailscale.com/util/testenv"
 	"tailscale.com/util/uniq"
+	"tailscale.com/util/usermetric"
 	"tailscale.com/wgengine/capture"
 	"tailscale.com/wgengine/wgint"
 )
@@ -386,6 +387,9 @@ type Options struct {
 	// report errors and warnings to.
 	HealthTracker *health.Tracker
 
+	// Metrics specifies the metrics registry to record metrics to.
+	Metrics *usermetric.Registry
+
 	// ControlKnobs are the set of control knobs to use.
 	// If nil, they're ignored and not updated.
 	ControlKnobs *controlknobs.Knobs