[{"group":"Alpha","test_1":{"description":"test description","value":1},"test_2":{"description":"test description","value":2},"test_3":{"description":"test description","value":3},"test_4":{"description":"test description","value":4},"test_5":{"description":"test description","value":5}}]

db.collection.aggregate([{$set:{kv:{$reduce:{input:{$objectToArray:"$$ROOT"},initialValue:[],in:{$concatArrays:["$$value",[{$cond:{if:{$eq:["$$this.k","group"]},then:{k:"$$this.k",v:"$$this.v"},else:{k:"$$this.k",v:{$ifNull:["$$this.v.value","$$this.v"]}}}}]]}}}}},{$replaceWith:{$arrayToObject:"$kv"}}])