$geoNear sorts documents by their distance from a given query point. Conceptually it both:
1. computes the distance, storing the result in the document metadata
2. sorts by that distance

The second step we can already express with $sort $meta.

Let's expose the first step as a new internal DocumentSource. The syntax might look something like this:

{$_internalComputeGeoNearDistance: {
  near: <point>,
  key: <fieldname>,
}}

Then a future ticket (SERVER-58602) can use this to allow $geoNear to execute anywhere in a pipeline.