Expose Aggregate.count(countName) method
Expose Aggregate.count(countName) method

The Aggregate class should expose a `count(countName)` method that adds a `$count` aggregator in the pipeline.

Documented here: http://mongoosejs.com/docs/api.html#aggregate_Aggregate-count
Code @5.0.15 referencing this method: https://github.com/Automattic/mongoose/blob/5.0.15/lib/aggregate.js#L379
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