Delta vCPU usage for all vCPUs, in vCPU-seconds.
To compute the per-vCPU utilization fraction, divide this value by (end-start)*N, where end and start define this value's time interval and N is `compute.googleapis.com/instance/cpu/reserved_cores` at the end of the interval.
This value is reported by the hypervisor for the VM and can differ from `agent.googleapis.com/cpu/usage_time`, which is reported from inside the VM.