Someone on reddit stated that snapd will update snaps regardless of what value `refresh.metered` holds when updates are postponed long enough. Unfortunately, I haven't been able to verify whether this claim is true or false.
Canonical should be up front about this type of information.
And I'm pretty sure NM lets you set all network connections metered, i.e.
See https://unix.stackexchange.com/questions/364927/networkmanag...