derp/xdp,cmd/xdpderper: initial skeleton (#12390)

This commit introduces a userspace program for managing an experimental
eBPF XDP STUN server program. derp/xdp contains the eBPF pseudo-C along
with a Go pkg for loading it and exporting its metrics.
cmd/xdpderper is a package main user of derp/xdp.

Updates tailscale/corp#20689

Signed-off-by: Jordan Whited <jordan@tailscale.com>

tsweb/promvarz/promvarz.go

@@ -28,7 +28,7 @@ func Handler(w http.ResponseWriter, r *http.Request) {
 // gatherNativePrometheusMetrics writes metrics from the default
 // metric registry in text format.
 func gatherNativePrometheusMetrics(w http.ResponseWriter) error {
-	enc := expfmt.NewEncoder(w, expfmt.FmtText)
+	enc := expfmt.NewEncoder(w, expfmt.NewFormat(expfmt.TypeTextPlain))
 	mfs, err := prometheus.DefaultGatherer.Gather()
 	if err != nil {
 		return fmt.Errorf("could not gather metrics from DefaultGatherer: %w", err)