Being that most of these routers are used with cellular connections, (USB modem, tethering, etc) this should be a requirement - as most cell data is metered. At least implement a way of manually tracking/resetting a data counter such as the one in luci under the Interface tab. Ideally with the ability to persist the count through interface up/down events such as disconnecting the cellular device or reboots. Even better would be a new function of the GL.iNet GUI, with the ability to push notifications, and set the monthly data cap and reset date.