tsweb: split promvarz into an optional dependency

Allows the use of tsweb without pulling in all of the heavy prometheus
client libraries, protobuf and so on.

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>

tsweb/promvarz/promvarz.go

@@ -11,12 +11,21 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/expfmt"
+	"tailscale.com/tsweb"
 	"tailscale.com/tsweb/varz"
 )
 
-// Handler returns Prometheus metrics exported by our expvar converter
+func init() {
+	tsweb.PrometheusHandler.Set(registerVarz)
+}
+
+func registerVarz(debug *tsweb.DebugHandler) {
+	debug.Handle("varz", "Metrics (Prometheus)", http.HandlerFunc(handler))
+}
+
+// handler returns Prometheus metrics exported by our expvar converter
 // and the official Prometheus client.
-func Handler(w http.ResponseWriter, r *http.Request) {
+func handler(w http.ResponseWriter, r *http.Request) {
 	if err := gatherNativePrometheusMetrics(w); err != nil {
 		w.WriteHeader(http.StatusInternalServerError)
 		w.Write([]byte(err.Error()))

tsweb/promvarz/promvarz_test.go

@@ -23,7 +23,7 @@ func TestHandler(t *testing.T) {
 	testVar1.Set(42)
 	testVar2.Set(4242)
 
-	svr := httptest.NewServer(http.HandlerFunc(Handler))
+	svr := httptest.NewServer(http.HandlerFunc(handler))
 	defer svr.Close()
 
 	want := `