A long time ago, we used to include a system monitor extension,
that added CPU/memory graphs to the (long gone) message tray.
However demand for this type of extensions hasn't died down, to the
point where RHEL includes a revived version of the old extension.
Account for that demand by adding a newly written system-monitor
extension that has been properly designed, and hopefully does not
bring back the CPU/memory issues of the previous one (unlikely
without the graphs) …
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/277>
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