We should add integration tests covering an extension stage that uses and produces metadata in a $unionWith pipeline. We should include cases like:

• reference metadata in top-level pipeline when extension stage in $unionWith provides metadata but top-level pipeline doesn't (error)
• reference metadata in top-level pipeline  when extension stage in $unionWith doesn't provide metadata but top-level pipeline does (error)
• reference metadata in top-level pipeline  when extension stage in $unionWith and top-level pipeline provide metadata (success)
• reference metadata in $unionWith pipeline when provided by extension stage (success) 
• extension stage in $unionWith pipeline references metadata that doesn't exist (error)
• etc