As you can see in the above picture, monitoring agent has to be installed to get memory utilization and disk space utilization, while CPU utilization and disk throughput do not need the agent.
Why is that? Cannot GCP gets memory utilization using the same method or tool that gets CPU utilization?
It's a matter of security and privacy. you can see the VM like a box. Google can see all the information outside of this box: Network throughput, disk throughput and CPU usage (number of cycle in fact).
But the content of the box is forbidden. You need to allow Google to look into the memory (what you use means check what you have write in it and thus get a read access to the memory. Potentially read more than only what is used and what is empty, and thus you need to autorize Google to do that). Same thing for the disk utilization.
