[{"field_1":{"subfield_1":{"subsubfield_1":"true","subsubfield_2":"false","subsubfield_3":"true"},"subfield_2":"sf2"},"field_2":{"subfield_1":{"subsubfield_1":"true","subsubfield_2":"false"},"subfield_2":"sf2"},"field_3":{"subfield_1":{"subsubfield_1":"true","subsubfield_2":"false","subsubfield_3":"false"},"subfield_2":"sf2"}}]

db.collection.aggregate([{$project:{doc:{$objectToArray:"$$ROOT"}}},{$unwind:"$doc"},{$match:{$expr:{$ne:["$doc.k","_id"]}}},{$project:{_id:0,subfield_1:{$size:{$filter:{input:{$objectToArray:"$doc.v.subfield_1"},cond:{$eq:["$$this.v","true"]}}}},subfield_2:"$doc.v.subfield_2"}}])