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/localapi/localapi.go

@@ -64,7 +64,6 @@ import (
 	"tailscale.com/util/progresstracking"
 	"tailscale.com/util/rands"
 	"tailscale.com/util/testenv"
-	"tailscale.com/util/usermetric"
 	"tailscale.com/version"
 	"tailscale.com/wgengine/magicsock"
 )
@@ -581,7 +580,7 @@ func (h *Handler) serveUserMetrics(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "usermetrics debug flag not enabled", http.StatusForbidden)
 		return
 	}
-	usermetric.Handler(w, r)
+	h.b.UserMetricsRegistry().Handler(w, r)
 }
 
 func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {

ipn/localapi/localapi_test.go

@@ -356,7 +356,7 @@ func newTestLocalBackend(t testing.TB) *ipnlocal.LocalBackend {
 	sys := new(tsd.System)
 	store := new(mem.Store)
 	sys.Set(store)
-	eng, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set, sys.HealthTracker())
+	eng, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set, sys.HealthTracker(), sys.UserMetricsRegistry())
 	if err != nil {
 		t.Fatalf("NewFakeUserspaceEngine: %v", err)
 	}