The serialize() and serializeToArray() methods of DocumentSource subclasses are used for too many purposes (and therefore could be quite complex):

• serializing pipeline stage for logging
• serializing pipeline stage for explain command output
• serializing pipeline stage for cloning or sending over the wire
• serializing pipeline stage for query shape hash computation

We can refactor explain-relevant parts into separate methods which can enrich the general-purpose serialization results with additional information. Doing this will make it more obvious:

• which information is needed by solely by explain and
• when this information is needed within the DocumentSource instance lifecycle