Upgrade from 16.04 to 18.04, indicator-multiload doesn't work
Solution 1:
Ubuntu 18.04 now use GNONE Shell instead of the Unity desktop like probably you already know. They are different desktops, but in general there are not limitations to use the same feature of Ubuntu 16.04 with Unity in Ubuntu 18.04 with GNOME Shell.
The same thing can be more easy or more hard to do in one or in the another desktop. This things you want, can be integrate better or worse with the rest of the shell to look like more or less pretty, or will work with worse or better performace, but finally it can be implemented in both desktops environments. A different history will not be true in general and need to be proven to then be taked as a true. Thats why i recommend be obtimistic. When something have not support, is not because a desktop is called X or Y, is because the developers behind the desktop are called X or Y. So, will be always a human factor and not a desktop name.
The case of the indicator-multiload is not different then and of course, it will work just fine with GNOME Shell. The problem was that the GNOME Shell developers, dosen' t likes the indicators, because this feature aparently do not match with his conception of the desktop. Source: https://bugzilla.gnome.org/show_bug.cgi?id=652122
Not matter if the shell developers deside to not support the indicators, because they really support extensions and then some thrid party developers can make possible the usage of the indicators inside the Gnome Shell session and thats what then happened when Jonas Kümmerlin create that extension: https://github.com/rgcjonas
When Ubuntu come to Gnome Shell in 18.04, the develop of the extension to loaded the indicators was not very active and was develop, preventing break the shell as possible, to cause few disturbs. So, it was limited a lot and some things was simplify in theory maybe, to be more faster and cause less problems.
So, the non-homogeneus indicators was then be loaded in a way that they will be converted to homogeneous indicators and then simplify the procedure to scale the icons inside panel. That was what the official shell code was doing in this time with the status icons. Please also see that the status icons was then be dropped for inside Gnome Shell: https://blogs.gnome.org/aday/2017/08/31/status-icons-and-gnome/ a few years ago, because the gnome shell developers droped his usage in favour of NOTHING and then do not considered the usage of the indicators again.
Also, some time ago, i found (https://github.com/ubuntu/gnome-shell-extension-appindicator/issues/41) this gnome shell extension. I forked and convert it to a Cinnamon applet and then I merged it with another Cinnamon oficial applet for the status icons. That code is inside Cinnamon right now (https://github.com/linuxmint/Cinnamon/pull/5790). In Cinnamon, the history was different, and i tested a lot, the possible ways to render the indicators and also i fix some bugs that the original extension already have, because the extension never was used with non homogeneus indicators. As an intresting thing, I can say that I never merged some of that improves in the Cinnamon desktop. Thats why in the cinnamon desktop the indicator-multiload is also homogeneus like it's in Ubuntu 18.04 right now.
When Ubuntu 18.04 was forked the indicator extension, i opened a bug there (https://github.com/ubuntu/gnome-shell-extension-appindicator/issues/92), to see if they want to fix something in the extension, but for what i see, they don't care to do so much. So, today i just forked the extension and fix the indicator-multiload only, because you want that, but don't think i will continue the develop of that extension or something similar. I think you need to ask to the ubuntu developers about that feature. Probably here: https://github.com/ubuntu/gnome-shell-extension-appindicator/issues/121
Edit: See that, I forked the extension and create a pull request for the version of the extension that was compatible with GNOME 3.28, but was not merged or adapted to be merged by someone else inside the official ubuntu extension: https://github.com/ubuntu/gnome-shell-extension-appindicator/pull/144
Solution 2:
On Ubuntu 18.04 indicator-multiload has bugs and causing problems. You should try to use "System Monitor Extension" instead. Detailed guide:
http://ubuntuhandbook.org/index.php/2019/03/display-cpu-memory-network-usage-in-ubuntu-18-04-panel/
As it's shows on this guide:
- First you install those packages:
sudo apt-get install gir1.2-gtop-2.0 gir1.2-networkmanager-1.0 gir1.2-clutter-1.0
- Then from Ubuntu Software Center search and install
system monitor extension.