Depending on which version of node exporter you are using, one of the following options will work.
Node exporter version go1.14.4
Node exporter version go1.15.8
Since the option “collector.system.unit-whitelist” was deprecated, the syntax has changed to “collector.systemd.unit-include”.
Note that there might be a bug in this version such that it won’t work if you put an equal (=) sign before the value: