$collStats is a new aggregation source in MongoDB 3.4. It provides a superset of the information available via the db.collection.stats() shell helper available in 3.2.

$collStats must be the first stage in a pipeline.

Synopsis:

db.collection.aggregate(
    { $collStats: 
        latencyStats: <document>,
        storageStats: {}
    }
)

Both latencyStats and storageStats are optional.

storageStats gives you storage-level statistics about a collection. It's exactly the same output as the old db.collection.stats() shell helper. This can only be specified on a collection; attempting it on a view results in the error CommandNotSupportedOnView.

latencyStats gives information about latencies for reads, writes and commands on a given namespace. It can be run on both views and collections. The latencyStats field must be a document that itself has one field, histograms. If histograms is true, the command returns the full operation latency histograms for that namespace. If false, the full histogram is omitted and only a summary is output.

*Note from allison.moore@mongodb.com*: We'll need to a reference page for the new $collStats pipeline operator.