Investigate changes in SERVER-54236: Implement $count accumulator

Description

{ $count: {} }

{ $sum: 1 }

Description of Linked Ticket

This task is for adding support for $count as an accumulator to the $group stage.

Scope of changes

New accumulator: https://docs.mongodb.com/manual/reference/operator/aggregation/#accumulators-group

Make sure we update https://docs.mongodb.com/manual/reference/operator/aggregation/group/#count-the-number-of-documents-in-a-collection as part of this change.

Also:

https://docs.mongodb.com/manual/reference/operator/aggregation/count/

Behavior section, current:

db.collection.aggregate( [

   { $group: { _id: null, myCount: { $sum: 1 } } },

   { $project: { _id: 0 } }

] )

Desired:

db.collection.aggregate( [

   { $group: { _id: null, myCount: { $count: {} } } },

   { $project: { _id: 0 } }

] )

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)