You can create custom metrics out of the existing ones, e.g.to calculate free space having total and occupied space, but what you're asking for (extracting an additional metric) is not possible in Oracle DB Insights.
With upcoming DB monitoring extensions though you'll be able to adapt the capabilities of an extension to your needs, including adding a new metric to be fetched. As a future alternative we also plan to make all Dynatrace supported extensions publicly available via git repository, so you'll also be able to easily request for new metrics using GitHub requests.
Empower Enterprise Apps and Services monitoring in Dynatrace
