tsweb: expose native Prometheus metrics in /debug/varz

The handler will expose built-in process and Go metrics by default,
which currently duplicate some of the expvar-proxied metrics
(`goroutines` vs `go_goroutines`, `memstats` vs `go_memstats`), but as
long as their names are different, Prometheus server will just scrape
both.

This will change /debug/varz behaviour for most tsweb binaries, but
notably not for control, which configures a `tsweb.VarzHandler`
[explicitly](a5b5d5167f/cmd/tailcontrol/tailcontrol.go (L779))

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

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

tsweb/tsweb.go

@@ -26,6 +26,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/common/expfmt"
 	"go4.org/mem"
 	"tailscale.com/envknob"
 	"tailscale.com/metrics"
@@ -672,6 +674,39 @@ func VarzHandler(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// CombinedVarzHandler is an HTTP handler for Prometheus metrics that
+// combines native metrics with the ones converted from expvar.
+func CombinedVarzHandler(w http.ResponseWriter, r *http.Request) {
+	if err := gatherNativePrometheusMetrics(w); err != nil {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte(err.Error()))
+		return
+	}
+	VarzHandler(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
+}
+
 // PrometheusMetricsReflectRooter is an optional interface that expvar.Var implementations
 // can implement to indicate that they should be walked recursively with reflect to find
 // sets of fields to export.