[inputs.systemd_units] Filter for multiple unittypes at the same time

[electrofloat]

Use Case

I want to use systemd_units to monitor both services AND sockets.

Expected behavior

I can use the unittype filter to add multiple types to it. For example: unittype = "service, socket"

Actual behavior

Currently only one type is accepted at the unittype

Additional info

I am maybe wrong and don't know Go enough, but it seems to me that only 1 unittype is accepted here:
https://github.com/influxdata/telegraf/blob/master/plugins/inputs/systemd_units/systemd_units_linux.go#L151