tsweb: move varz handler(s) into separate modules

This splits Prometheus metric handlers exposed by tsweb into two
modules:
- `varz.Handler` exposes Prometheus metrics generated by our expvar
  converter;
- `promvarz.Handler` combines our expvar-converted metrics and native
  Prometheus metrics.

By default, tsweb will use the promvarz handler, however users can keep
using only the expvar converter. Specifically, `tailscaled` now uses
`varz.Handler` explicitly, which avoids a dependency on the
(heavyweight) Prometheus client.

Updates https://github.com/tailscale/corp/issues/10205

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

tsweb/promvarz/promvarz.go

@@ -0,0 +1,48 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package promvarz combines Prometheus metrics exported by our expvar converter
+// (tsweb/varz) with metrics exported by the official Prometheus client.
+package promvarz
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/common/expfmt"
+	"tailscale.com/tsweb/varz"
+)
+
+// Handler returns Prometheus metrics exported by our expvar converter
+// and the official Prometheus client.
+func Handler(w http.ResponseWriter, r *http.Request) {
+	if err := gatherNativePrometheusMetrics(w); err != nil {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte(err.Error()))
+		return
+	}
+	varz.Handler(w, r)
+}
+
+// gatherNativePrometheusMetrics writes metrics from the default
+// metric registry in text format.
+func gatherNativePrometheusMetrics(w http.ResponseWriter) error {
+	enc := expfmt.NewEncoder(w, expfmt.FmtText)
+	mfs, err := prometheus.DefaultGatherer.Gather()
+	if err != nil {
+		return fmt.Errorf("could not gather metrics from DefaultGatherer: %w", err)
+	}
+
+	for _, mf := range mfs {
+		if err := enc.Encode(mf); err != nil {
+			return fmt.Errorf("could not encode metric %v: %w", mf, err)
+		}
+	}
+	if closer, ok := enc.(expfmt.Closer); ok {
+		if err := closer.Close(); err != nil {
+			return err
+		}
+	}
+	return nil
+}

tsweb/promvarz/promvarz_test.go

@@ -0,0 +1,35 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+package promvarz
+
+import (
+	"expvar"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/prometheus/client_golang/prometheus/testutil"
+)
+
+func TestHandler(t *testing.T) {
+	test1 := expvar.NewInt("gauge_promvarz_test_expvar")
+	test1.Set(42)
+	test2 := promauto.NewGauge(prometheus.GaugeOpts{Name: "promvarz_test_native"})
+	test2.Set(4242)
+
+	svr := httptest.NewServer(http.HandlerFunc(Handler))
+	defer svr.Close()
+
+	want := `
+	# TYPE promvarz_test_expvar gauge
+	promvarz_test_expvar 42
+	# TYPE promvarz_test_native gauge
+	promvarz_test_native 4242
+	`
+	if err := testutil.ScrapeAndCompare(svr.URL, strings.NewReader(want), "promvarz_test_expvar", "promvarz_test_native"); err != nil {
+		t.Error(err)
+	}
+}