If you need the granular data of per session then it's custom code, db to store data and API calls from openvpn, if it even exists for openvpn.
Nagios could give you per IP bandwidth usage. Keep in mind, if someone connects from a router, a cafe or college public IP then many people usage will be under 1 IP. So you won't get a per session number.
If somehow the newest version of Nagios has this capability please let the community here know as I've moved on from that utility long ago.