[{"_id":"1","ML_PRIORITY":"HIGH","STATUS":"True"},{"_id":"2","ML_PRIORITY":"HIGH","STATUS":""},{"_id":"3","ML_PRIORITY":"HIGH","STATUS":"False"},{"_id":"4","ML_PRIORITY":"MEDIUM","STATUS":""},{"_id":"5","ML_PRIORITY":"Low","STATUS":""}]

db.collection.aggregate([{"$group":{"_id":"$ML_PRIORITY","QUALITYCHECKDONE":{"$sum":{"$cond":[{"$eq":["$STATUS","TRUE"]},1,0]}},"QUALITYCHECKNOTDONE":{"$sum":{"$cond":[{"$eq":["$STATUS",""]},1,0]}},"QUALITYCHECKNOTREQ":{"$sum":{"$cond":[{"$eq":["$STATUS","FALSE"]},1,0]}}}},{$set:{order:{$indexOfArray:[["HIGH","MEDIUM","Low"],"$_id"]},"ML_PRIORITY":"$_id"}},{$sort:{order:1}},{$unset:["_id","order"]}])