cgroups should not be checked if substate is exited

[JensErat]

The Context

As described in #12, substates are often more describing. For example, we're using cloud-init, which has a unit of Type=oneshot. Those run a single time, and afterwards have ActiveState=active but SubState=exited.

Example unit:

$ systemctl status cloud-final
● cloud-final.service - Execute cloud user/final scripts
   Loaded: loaded (/lib/systemd/system/cloud-final.service; enabled; vendor preset: enabled)
   Active: active (exited) since Thu 2020-05-07 07:03:55 UTC; 1 weeks 6 days ago
  Process: 1118 ExecStart=/usr/bin/cloud-init modules --mode=final (code=exited, status=0/SUCCESS)
 Main PID: 1118 (code=exited, status=0/SUCCESS)
    Tasks: 0
   Memory: 544.9M
   CGroup: /system.slice/cloud-final.service

The Issue

systemd_exporter tries to scrape cgroup information for this unit. From reading the code, I see this switch/case statement must be extended to exclude some substates (or probably better: include only really active ones, #12 has a more comprehensive list that can be used to derive): https://github.com/povilasv/systemd_exporter/blob/master/systemd/systemd.go#L578

[povilasv]

Thanks for reporting.