collectd installed by reForis does not start collecting statistics
If you install from reForis collectd (Statistics) everything necessary is installed except the collectd-mod-load package. Since the "load" parameter is active in the configuration, monitoring is not activated and no rrd file is created. So either you have to automatically install that package as well or you have to remove that parameter from the default configuration. Only by searching the OpenWrt forum did I find the solution.