Returns an aggregation of the bottom n elements within a group, according to the specified sort order.
If the group contains fewer than n elements, $bottomN returns all elements in the group.
Param: collection
The input array
Param: expr
The right-hand side expression value of the operator
Returns an aggregation of the bottom n elements within a group, according to the specified sort order. If the group contains fewer than n elements, $bottomN returns all elements in the group.