Monitor GPU usage by Virtual Machine

[vickmp]

Description
Add the GPU usage of a Virtual Machine to the metrics collected by the OpenNebula monitoring probes, in the same way CPU, memory and disk metrics are implemented.

Use case
Adding GPU usage metrics can help to better monitor GPU-based processes instead of CPU-based processes, such as graphics loads or AI or ML processes.

Additional context
It's important to note that not all VMs have GPUs (as is the case with CPU, memory and disk), so this should be taken into account when processing VM information.

Progress Status

• Code committed
• Testing - QA
• Documentation (Release notes - resolved issues, compatibility, known issues)