Database
[{"A":"1","B":"3","C":"0.000090213",},{"A":"1","B":"3","C":"0.000037698",},{"A":"2","B":"4","C":"0.00016664",}]
Query
db.collection.aggregate([{$facet:{"A":[{$group:{_id:"$A",v:{$sum:1}}}],"B":[{$group:{_id:"$B",v:{$sum:1}}}]}},{$addFields:{A:{$map:{input:"$A",as:"a",in:{k:"$$a._id",v:"$$a.v"}}}}},{$addFields:{B:{$map:{input:"$B",as:"b",in:{k:"$$b._id",v:"$$b.v"}}}}},{$project:{A:{"$arrayToObject":"$A"},B:{"$arrayToObject":"$B"}}}])