The pipeline class is currently missing methods associated with the new-in-MongoDB-3.4 pipeline stages:

• $addFields
• $bucket
• $bucketAuto
• $count
• $collStats
• $facet
• $graphLookup
• $replaceRoot
• $sortByCount

It is also missing a method associated with the $indexStats stage (new in MongoDB 3.2), and the $geoNear stage.

In addition, pipeline::unwind() needs a new overload that allows users to specify values for the "includeArrayIndex" and "preserveNullAndEmptyArrays" options (new in MongoDB 3.2).

We should either add new builder methods to the pipeline class for all of these new stages, or we should deprecate the pipeline class entirely and instead have users pass array builders directly to collection::aggregate().