Preencher + Agregar no Mongoose

// Your aggregate query from your question
Transaction.aggregate([{
                            $unwind: '$tags'
                        }, {
                            $group: {
                                _id: '$tags',
                                amount: {
                                    $sum: '$amount'
                                }
                            }
                        }])
    .exec(function(err, transactions) {
        // Don't forget your error handling
        // The callback with your transactions
        // Assuming you are having a Tag model
        Tag.populate(transactions, {path: '_id'}, function(err, populatedTransactions) {
            // Your populated translactions are inside populatedTransactions
        });
    });