Don’t give up 
I did a test on the VM.
I entered the following lines into the /etc/collectd.custom file and rebooted the VM.
The result, after entering into the /etc/collectd.custom file, rebooting and waiting a few minutes
Note that the interval is 30 seconds.
edit:
Next test.
I edited the /etc/collectd.custom file as follows and restarted the VM.
Below is the effect after rebooting and waiting a few minutes.
edit2:
Next test just to be sure.
Tested on Core Update 169 Development Build
Regards.