Cube metadata will contain separate 'local' + 'global' attribute dicts.
Effectively, the 'local' is the existing dict, named "metadata.attributes', while the 'metadata.global_attributes' is a new one.

Nothing special links the two (at least initially).
There may be future scope for additional behaviour that understands the connection between the two (e.g. a lenient type service)

Related task #4983 will specialise access to "cube.attributes" to enable a unified access to both dicts, or to each specifically.