MongoDB Aggregation: How to pick first doc from each bucket?

Working with Data
1

My aggregation stages look like:

[
  { $match: { ...someFilters } },
  { $sort: { createdOn: -1 } },
  { 
    $bucket: { 
      groupBy: $somePropertyInDoc,
      output: {
        DO_SOMETHING_TO_PICK_FIRST_DOC_IN_THE_BUCKET
      }
    } 
  }
]

I am assuming that buckets are arrays of documents which are transformed based on what is specified in the output property.
Is there any way to pick/output the first document from each bucket?