This ticket should add or extend test extensions to confirm that metadata reporting and metadata execution works as intended. Ideas for test cases:

1. Metadata on a source stage
2. Metadata on a transform stage
3. Metadata on a transform stage after an extension source/transform stage.
4. Extension adding metadata it didn't report (allowed)
5. Extension deleting metadata (allowed)
6. Test $sortKey with multi key sort.

Please feel free to split up this ticket if necessary.