In order to support $rerank, we will need to add a new metadata variable that can be used to represent the $rerank score. 

Adding arbitrary new metadata fields is a non-goal of this project, due to the complexity of metadata fields the document class. Metadata fields are kept in a bitfield within the document class, so adding new arbitrary metadata variables specified by an extension will be complicated. 

For now, we will simply introduce a new metadata variable for $rerank score in the server code, so that $rerank can use it when it is implemented as an extension.