agg.js

db.website_performance.aggregate(
[
    { 
      $match : { 'domain'  : { $in: []} }
    }, 
    {
      $group: { 
            _id: "$domain", 
            sum_impressions: { $sum: "$impressions" },
            sum_bids: { $sum: "$bids" } }
    },
    { 
      $project: {
            ctr: { $multiply: [ 100, { $divide: [ "$sum_impressions", "$sum_bids" ] } ]
            }
        }
    },
    {
        $sort: {
            ctr: -1
        }
    },
    {
        $limit: 0
    }
]);