[{"cpnj":"someValue","bairro":10,"periodos":{"2017":{"jan":1,"feb":10,"mar":10},"2018":{"jan":11,"feb":101,"mar":101},"2019":{"jan":1,"feb":1,"mar":101}}},{"cpnj":"someValue3","bairro":1011,"periodos":{"2019":{"jan":1,"feb":0,"mar":10}}},{"cpnj":"someValue4","bairro":10111,"periodos":{"2020":{"jan":1,"feb":0,"mar":10}}},{"cpnj":"someValue2","bairro":101,"periodos":{"2017":{"jan":1,"feb":0,"mar":10}}}]

db.collection.aggregate([{$addFields:{periodos:{$objectToArray:"$periodos"}}},{$match:{"periodos.k":"2017"}},{$addFields:{periodos:{$arrayToObject:"$periodos"}}}])