To help prevent something like SERVER-99887 in the future. During that code review we thought we could use some more integration tests surrounding other kinds of metadata, referenced by the user, for example:

db.example.aggregate([
  {$geoNear: {/* ... */}},
  {$setWindowFields: {/* ... */}}, 
  {$project: {distance: {$meta: "geoNearDistance"}}}
])

or a similar one with $search and 'searchScore' meta, and whatever else comes to mind.

In particular, the test should be sure to stress whether spilling to disk causes any issues, since that was the case that gave us trouble in SERVER-99887.