As per the product documentation the Capacity Forecasting is not available for non percentile metrics in the Capacity Analytics and these are only available in DX Dashboards.
It would be better if the non percentile metrics (Example: Database size in MB) are available in Capacity Analytics. Can this be added as an enhancement request?
Release : 20.2
Component : CA DOI CAPACITY PREDICTIVE ANALYTICS
For non-percentile metrics forecasting, we suggest to use the DX-Dashboards.
For enhancement request forecasting non-percentile metrics in the Capacity Analytics is under review.