I have installed indicator-multiload, but the bars does not display correctly and flickers in a weird way (width expands and contracts quickly).
I have reduced the number of desktops to 1 in the hope that this fixes the problem, but it did not work. The top bar tray is part of {Extension Manager}/{Ubuntu Appindicators} and can be hidden by toggling that off.
It seems to be related to an existing bug: https://bugs.launchpad.net/ubuntu/+source/indicator-multiload/+bug/1974063
After I upgraded to 22.04 I tried indicator-multiload and find it quite annoying with all the flickering on the menubar and not particularly good looking. Changing your desktop environment just for that is just not worth it.
I was using gnome-shell-extension-system-monitor in 20.04 and I was happy with it, but it is not yet ported to 22.04 and does not work.
After some searching I found another gnome shell extension and first impressions are very, very positive - TopHat. Feel free to try it out.
As @Stefan Buynov mentioned, tophat really is a stunning project.
In Ubuntu 22.04 if someone still wants to try gnome-shell-extension-system-monitor, there is a fork project system-monitor-next supporting Gnome 42. It's already uploaded to the Gnome extension store and has the same description as the system-monitor so I found it.
For now, the most convenient way to install it is using extension-manager.
sudo apt install gnome-shell-extension-manager, it's not the latest version but it's ok.
You can open it by running extension-manager in the command line or find it in Show Applications.
You can also use it to install tophat.
In the end, an idea came into my mind that GitHub should make the fork list of a repo more informative, so in the fork list of an unmaintained project, we may find someone keeping the maintaining job on. Not only that, some fork projects get more attention than the original ones.
I found a Chrome extension Useful Forks that implements this function for GitHub.
Initially thought about leaving a comment, as it is not the exact solution for the question, but using Unity instead of Gnome on my desktop has proven to be a great experience. Not only the indicator work, but the whole seem to run more smoothly (Gnome / shell seem to be quite buggy).
Also, while Unity can be installed and used, one can switch back to Gnome from the login screen if necessary (unless you remove it of course).
Install Unity (and the indicators if not already)
sudo apt install ubuntu-unity-desktop indicator-multiload
Reboot.
Unity should come as default (or select it from the login screen).
