You don’t provide any scenario requiring monitoring, so this is a generic answer. As a low impact home solution, you can make a simple queue to monitor your gateway interface and reset it manually each month. You can make a queue for each user/client, and do the same thing. The problem with services running on small embedded devices is always overhead. Many brands offer some sort of monitoring with a fancy GUI, but there’s a cost to it, one way or another. For professional/lab applications, having an external device/server isn’t generally an issue. MT has The Dude, but there are other solutions for ISP/WISP operators. Lots of threads on Reddit. If you are inquiring for a commercial application, consultants are also available.
http://forum.mikrotik.com/t/tool-using-splunk-to-analyse-mikrotik-logs-4-0-graphing-everything/153043/1
http://forum.mikrotik.com/t/to-mt-keep-accounting-v7-x/151083/1
https://docs.librenms.org/Support/Device-Notes/Routeros/