know the free space of a persistent disk

[sudheerdon]

I'm looking for api which can determine the frees space of a persistent disk.

I have checked this but no proper attributes for determine the free space of a disk.

https://cloud.google.com/compute/docs/reference/rest/v1/disks

Ex: I have a disk which is size of 100GB but internal the OS/data was consumed only 60GB and there is 40GB free to use. I want to automate to add more disks if it hits the 90% of storage provisioned.