wgengine/magicsock: packet/bytes metrics should not count disco

Updates #13420

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -1267,7 +1267,7 @@ func (c *Conn) sendUDPBatch(addr netip.AddrPort, buffs [][]byte) (sent bool, err
 
 // sendUDP sends UDP packet b to ipp.
 // See sendAddr's docs on the return value meanings.
-func (c *Conn) sendUDP(ipp netip.AddrPort, b []byte) (sent bool, err error) {
+func (c *Conn) sendUDP(ipp netip.AddrPort, b []byte, isDisco bool) (sent bool, err error) {
 	if runtime.GOOS == "js" {
 		return false, errNoUDP
 	}
@@ -1276,7 +1276,7 @@ func (c *Conn) sendUDP(ipp netip.AddrPort, b []byte) (sent bool, err error) {
 		metricSendUDPError.Add(1)
 		_ = c.maybeRebindOnError(runtime.GOOS, err)
 	} else {
-		if sent {
+		if sent && !isDisco {
 			switch {
 			case ipp.Addr().Is4():
 				c.metrics.outboundPacketsIPv4Total.Add(1)
@@ -1371,7 +1371,7 @@ func (c *Conn) sendUDPStd(addr netip.AddrPort, b []byte) (sent bool, err error)
 // returns (false, nil); it's not an error, but nothing was sent.
 func (c *Conn) sendAddr(addr netip.AddrPort, pubKey key.NodePublic, b []byte, isDisco bool) (sent bool, err error) {
 	if addr.Addr() != tailcfg.DerpMagicIPAddr {
-		return c.sendUDP(addr, b)
+		return c.sendUDP(addr, b, isDisco)
 	}
 
 	regionID := int(addr.Port())