feature/*,net/tstun: add tundev_txq_drops clientmetric on Linux

By polling RTM_GETSTATS via netlink. RTM_GETSTATS is a relatively
efficient and targeted (single device) polling method available since
Linux v4.7.

The tundevstats "feature" can be extended to other platforms in the
future, and it's trivial to add new rtnl_link_stats64 counters on
Linux.

Updates tailscale/corp#38181

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

net/tstun/wrap.go

@@ -24,6 +24,7 @@ import (
 	"go4.org/mem"
 	"tailscale.com/disco"
 	"tailscale.com/envknob"
+	"tailscale.com/feature"
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/net/packet"
 	"tailscale.com/net/packet/checksum"
@@ -222,6 +223,10 @@ type Wrapper struct {
 
 	eventClient              *eventbus.Client
 	discoKeyAdvertisementPub *eventbus.Publisher[events.DiscoKeyAdvertisement]
+
+	// tunDevStatsCloser closes TUN device stats polling. It may be nil if
+	// [HookPollTUNDevStats] is unset, or the hook func returned an error.
+	tunDevStatsCloser io.Closer
 }
 
 type metrics struct {
@@ -296,6 +301,16 @@ func wrap(logf logger.Logf, tdev tun.Device, isTAP bool, m *usermetric.Registry,
 		metrics:     registerMetrics(m),
 	}
 
+	if buildfeatures.HasTUNDevStats {
+		if f, ok := HookPollTUNDevStats.GetOk(); ok {
+			closer, err := f(tdev)
+			if err != nil {
+				w.logf("error initializing tun dev stats polling: %v", err)
+			}
+			w.tunDevStatsCloser = closer
+		}
+	}
+
 	w.eventClient = bus.Client("net.tstun")
 	w.discoKeyAdvertisementPub = eventbus.Publish[events.DiscoKeyAdvertisement](w.eventClient)
 
@@ -313,6 +328,9 @@ func wrap(logf logger.Logf, tdev tun.Device, isTAP bool, m *usermetric.Registry,
 	return w
 }
 
+// HookPollTUNDevStats is the hook maybe set by feature/tundevstats.
+var HookPollTUNDevStats feature.Hook[func(dev tun.Device) (io.Closer, error)]
+
 // now returns the current time, either by calling t.timeNow if set or time.Now
 // if not.
 func (t *Wrapper) now() time.Time {
@@ -374,6 +392,9 @@ func (t *Wrapper) Close() error {
 		t.outboundMu.Unlock()
 		err = t.tdev.Close()
 		t.eventClient.Close()
+		if t.tunDevStatsCloser != nil {
+			t.tunDevStatsCloser.Close()
+		}
 	})
 	return err
 }