Returns an aggregation of the maxmimum value n elements within a group.
If the group contains fewer than n elements, $maxN 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 maxmimum value n elements within a group. If the group contains fewer than n elements, $maxN returns all elements in the group.