Given that we want to extend the metrics available through the Metrics API, and that Kubernetes allows us that through it's Custom Metrics API (https://github.com/kubernetes/metrics/tree/master/pkg/apis/custom_metrics), one option to accomplish our goals could be to create our own adapter.
Depending on the application (DB) where we store metrics, that might be a good option. But, given that it is pointless to reinvent the wheel, our first step should be to search for a solution. If someone already created an adapter that suits our needs, it would make sense to adopt it, instead of creating a new one by ourselves. Even if we do choose something that provides only part of the features we're looking for, it's easier to build on top of it (and contribute back to the project), than to start from scratch.
Given that our metrics are stored...