With Ubuntu 18.04 LTS switching back to Gnome as the desktop manager from Unity, it means it can take advantage of Gnomes features that replaced unities such as system monitor in the dock.
Open the gnome terminal and type the command:
sudo apt install gnome-shell-extension-system-monitor
After the install is complete, restart the computer.
For additional Gnome Shell Extensions and management of Gnome Shell, install Tweaks which includes a UI to customise Gnome Shell:
sudo apt install gnome-shell-extensions
Any extensions installed will require a restart before the appear in Tweaks.
I wish Tweaks app was built into Gnome settings. Gnome Shell Extensions on the front end looks the same as Gnome applets with the addition of Tweaks to manage them.