This doesn't answer your question, but is similar. Compute Cluster Total disk space includes NFS, Template, etc, datastores that are not used for VM placement. This super metric includes only datastores that are used by VMs.
Object type = compute cluster resource
floor(sum(${adaptertype=VMWARE, objecttype=Datastore, metric=capacity|total_capacity, depth=2, where= (${RESOURCE_NAME} startsWith 'HPEDatastore' || ${RESOURCE_NAME} startsWith 'EMCDatastore') })/1024)