wgengine/magicsock: exclude disco from throughput metrics

The user-facing metrics are intended to track data transmitted at
the overlay network level.

Updates tailscale/corp#22075

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

wgengine/magicsock/endpoint.go

@@ -983,7 +983,8 @@ func (de *endpoint) send(buffs [][]byte) error {
 		allOk := true
 		var txBytes int
 		for _, buff := range buffs {
-			ok, _ := de.c.sendAddr(derpAddr, de.publicKey, buff)
+			const isDisco = false
+			ok, _ := de.c.sendAddr(derpAddr, de.publicKey, buff, isDisco)
 			txBytes += len(buff)
 			if !ok {
 				allOk = false