Last thing which was missing was to add new router to monitoring using Munin which I use for this purpose. The only reasonable way seemed to be SNMP so I installed it to OpenWrt.
And the rest was incredibly simple! Just add snmp host to
[openwrt.lan] address localhost
And run SNMP discovery on monitoring host (the one which runs munin-node):
munin-node-configure-snmp openwrt.lan | sh
It automatically creates symlinks for everything it detects (network interfaces, load, disk space and users) and only remaining thing is to restart munin-node.