Sub items:

• Extend the each url with 'size' information
• Extend the $unionWith IDL: document_source_union_with.idl
• When we see an external data source definition in $unionWith’s coll field, we create  placeholder collection(s) temporarily inside the ‘DocumentSourceUnionWith’ class. Each url will be mapped to a separate placeholder collection. These placeholder collections will be destroyed when the ‘DocumentSourceUnionWith’ object is destroyed. The placeholder collections can be created by calling ‘createCollection’ command with an auto-generated collection name on ‘DBDirectClient’ object. When a placeholder collection is created, information about the external data source is passed to the underlying ‘WiredTigerRecordStore’ object.