tsweb: add num_goroutines expvar

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tsweb/tsweb.go

@@ -42,6 +42,7 @@ func NewMux(debugHandler http.Handler) *http.ServeMux {
 
 func registerCommonDebug(mux *http.ServeMux) {
 	expvar.Publish("counter_uptime_sec", expvar.Func(func() interface{} { return int64(Uptime().Seconds()) }))
+	expvar.Publish("gauge_goroutines", expvar.Func(func() interface{} { return runtime.NumGoroutine() }))
 	mux.Handle("/debug/pprof/", Protected(http.DefaultServeMux)) // to net/http/pprof
 	mux.Handle("/debug/vars", Protected(http.DefaultServeMux))   // to expvar
 	mux.Handle("/debug/varz", Protected(http.HandlerFunc(VarzHandler)))