feature: add status change timestamp

[Janek Lauer]

A feature of e.g. podman ps -a is to show the relative time of the last state change (Up 3 minutes ago).

Would an additional gauge with this timestamp be possible?

Right now I can't detect restart loops because the state is not exited long enough for prometheus to notice.

Thanks for this project! :)

[Navid Yaghoobi]

Hi @schlauerlauer
Thanks for reaching.
#22 will cover this feature request.