[{_id:1,field1:"value",field2:"a",},{_id:2,field1:"value",field2:"A",},{_id:3,field1:"value",field2:"C",},{_id:4,field1:"value",field2:"b",}]

db.collection.aggregate([{$group:{_id:{$toUpper:"$field2"},count:{$sum:1}}},{$group:{_id:null,counts:{$push:{k:"$_id",v:"$count"}}}},{$replaceWith:{$arrayToObject:"$counts"}}])