There are different ways to monitor free space. I hope that I have understood you correctly, so that one of the suggested ways would be exactly what you're after:
1. Monitoring free space of the datastore. This can be done via predefined alarm for datastores and via these reports:
2. Monitoring used space of VM disks. This can be achieved via reviewing the report below:
3. Monitoring guest OS disk space. This is possible via predefined alarm (for guest disk space) and the corresponding report:
Based on what you've said above, you want to monitor virtual disk space usage and it seems like you're using thin provisioning. Is that correct?
If not and you're using thick disks (type), then free space for these disks will be the same as guest OS volumes free space, so an option 3 should be a way to go.