metering data usage on one port?

Hi, I’be got an RB1100AH and I want to assign ether12 to be solely used by a separate office space. i’m looking for a way to then meter (and bill) any data usage on this port, loving usage over time so I can look at a summary each month.

Whats the best approach here? I’ve been reading about torch and some custom scripts but I think the custom scripts rely on a hotspot or user authentication. Is there anything that can just measure data usage on one port?

Thanks!

Read data by snmp and store them on different computer. Otherwise you will loose values each time the router reboots.