Raw containers ( such as systemd services ) don't have main PID, so
before this change they weren't allowed to retrieve various stats from
the cgroup such as networking stats.
However, some process stats such as file descriptor counts or number of
processes are still valuable to those containers and we should be able
to retrieve them without depending on the main pid.
For example, on my laptop:
Before:
```
container_processes{id="/system.slice/containerd.service"} 0 1593123685900
```
After:
```
container_processes{id="/system.slice/containerd.service"} 4 1593123707235
```
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