You can monitor your openvpn log files very easily with Splunk.
Your openvpn log file locations are specified in /etc/openvpn/server.conf
On my system for example the log files are /var/log/openvpn.log and /var/log/openvpn-status.log
The openvpn-status log file has a list of open client connections , so you could monitor this :
