feature/featuretags: make usermetrics modular

Saves ~102 KB from the min build.

Updates #12614

Change-Id: Ie1d4f439321267b9f98046593cb289ee3c4d6249
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/debug.go

@@ -28,7 +28,9 @@ import (
 	"tailscale.com/ipn"
 	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tsweb/varz"
 	"tailscale.com/types/key"
+	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/eventbus"
 )
 
@@ -58,6 +60,12 @@ func newDebugMux() *http.ServeMux {
 	return mux
 }
 
+func servePrometheusMetrics(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/plain")
+	varz.Handler(w, r)
+	clientmetric.WritePrometheusExpositionFormat(w)
+}
+
 func debugMode(args []string) error {
 	fs := flag.NewFlagSet("debug", flag.ExitOnError)
 	fs.BoolVar(&debugArgs.ifconfig, "ifconfig", false, "If true, print network interface state")