To truly get a picture of your virtual infrastructure, you need to get metrics from the virtual platform and not from the VMs themselves. This said, there is merit to getting performance metrics from within the VM guest operating systems as well - for example, if a VM is taking up high CPU, why is it - load? malware? etc. As a VMware consultant, i recommend getting metrics from both within and outside the VM and making intelligent decisions. For example, see the discussion here -
http://www.eginnovations.com/web/vmware-technology.htm
Regards,
Simon L.