Add render slot for gap area in Metric

[gvnmagni]

Is your feature request related to a problem? Please describe.
Solutions teams have requested a way to consistently place tags below the title on the Metric component. This is not easy to force externally with consistency.

Describe the solution you'd like
Ideally we allow a render prop on the Metric chart that we pass each datum to in order to render the contents into the gap area.

The user would be responsible for padding and sizing of the gap content as applicable to their use case.

Describe alternatives you've considered
Rendering custom elements absolutely relative to the Metric chart. Causes inconsistent placement of contents.

Additional context
Since the Metric uses dom elements this approach is preferred over dimensional placement of the render contents.