Add an option to the $sample stage to specify weights to use in the sampling.

Specifying this option would prevent any optimized random cursor implementation from the storage engine, and would always use a top-k random sort, with the random value used to sort being multiplied by the specified weight.

For example:

db.example.aggregate([{

    $sample: {

        size: 100,

        weight: "$myWeightField"

    }

}]);