Some services expose extensions that allow their schemas to be expanded in ways they ultimately need to know about. Consider an access management interface (like RBAC) that exposes extensions for dependent services to register permissions, and then needs to know what those were in order to populate UIs. 

The idea here is to be able to capture metadata in the schema that can then be exposed to service providers. In the immediate term, it should allow RBAC to switch to using Kessel schema features to replace the permissions (and potential roles) JSONs in rbac-config

Implementation details will likely be impacted by Transformation/Delivery (RHCLOUD-44314): with incremental deployments, inventory would likely be responsible for detecting and publishing metadata changes (likely as events), while monolithic deployments may allow for more options.