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/debug.go

@@ -14,7 +14,7 @@ import (
 	"os"
 	"runtime"
 
-	"tailscale.com/tsweb/promvarz"
+	"tailscale.com/feature"
 	"tailscale.com/tsweb/varz"
 	"tailscale.com/version"
 )
@@ -37,6 +37,11 @@ type DebugHandler struct {
 	title    string                           // title displayed on index page
 }
 
+// PrometheusHandler is an optional hook to enable native Prometheus
+// support in the debug handler. It is disabled by default. Import the
+// tailscale.com/tsweb/promvarz package to enable this feature.
+var PrometheusHandler feature.Hook[func(*DebugHandler)]
+
 // Debugger returns the DebugHandler registered on mux at /debug/,
 // creating it if necessary.
 func Debugger(mux *http.ServeMux) *DebugHandler {
@@ -53,7 +58,11 @@ func Debugger(mux *http.ServeMux) *DebugHandler {
 	ret.KVFunc("Uptime", func() any { return varz.Uptime() })
 	ret.KV("Version", version.Long())
 	ret.Handle("vars", "Metrics (Go)", expvar.Handler())
-	ret.Handle("varz", "Metrics (Prometheus)", http.HandlerFunc(promvarz.Handler))
+	if PrometheusHandler.IsSet() {
+		PrometheusHandler.Get()(ret)
+	} else {
+		ret.Handle("varz", "Metrics (Prometheus)", http.HandlerFunc(varz.Handler))
+	}
 
 	// pprof.Index serves everything that runtime/pprof.Lookup finds:
 	// goroutine, threadcreate, heap, allocs, block, mutex

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 := `