The two are related to each other.
Alternative is to use a simpler channel like HTTP. SNMP is sometimes too big/bloated for small routers.
Collectd is a nice little program that is able to collect various data on a system, and either write them to a rrdtool database, or send them via the network (or both).
It runs on OpenWRT: statistic.collectd. This is quite nice with the network-based reporting of the collected data (because you probably don't want to store rrdtool database directly on your OpenWRT routers).