Add a client metric to observe whether we can clean up IPV6 NAT check

### What are you trying to do?

See https://github.com/tailscale/tailscale/pull/11353#discussion_r1515181498 for context, it would be good if we could clean up that part of code and remove the usage of modprobe. Having a metric to tell us whether the nat table listing is sufficient would allow us to safely remove the other checks.