not true anymore, since HA Core 2023.4 CPU + MEM are also available for Core + Supervisor:
The Supervisor integration now provides sensors containing the Home Assistant Core and Supervisor’s stats. Thanks, @ludeeus!
Very convenient option, in my opinion really the best way to go. Unfortunately it seems to increase overall system CPU usage significantly, not sure if it’s a general or a “for my system only” issue. For details see
How to access core and supervisor CPU + MEM usage (HA OS) - #7 by e-raser