Add faceting

[erikhatcher]

Facets would be returned within the meta response.

[erikhatcher]

If faceting is on, limit=0 is allowed (return facets, but no documents) - will this work with Atlas Search?

[erikhatcher]

If faceting is on, limit=0 is allowed (return facets, but no documents) - will this work with Atlas Search?

Or switch to $searchMeta when limit=0.