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>

util/usermetric/usermetric_test.go

@@ -9,7 +9,8 @@ import (
 )
 
 func TestGauge(t *testing.T) {
-	g := NewGauge("test_gauge", "This is a test gauge")
+	var reg Registry
+	g := reg.NewGauge("test_gauge", "This is a test gauge")
 	g.Set(15)
 
 	var buf bytes.Buffer