You can get all the available metrics through the Get-Stat cmdlet.
With Get-StatType you can find all available metrics for an entity in a specific interval.

Get-Stat -Entity $vm -Disk

Get-Stat The metric counter "disk.write.average" doesn't exist for entity "vm-test".

Get-Stat The metric counter "disk.read.average" doesn't exist for entity "vm-test".

Get-Stat The metric counter "disk.write.average" doesn't exist for entity "vm-test".

Those metrics require statistics level 2, is that configured on your VCSA?
Or are you querying Realtime statistics? Did you add that switch?
With just copying the error message there isn't a lot I can help with