Database
[{server:{_id:"6108d87b9255993b26796ca9",version:"2.0.366",name:"aaaaaa",variables:[{key:"aKey",value:"aVal",},{key:"bKey",value:"bVal",},{key:"cKey",value:"cVal",}],__v:0}}]
Query
db.collection.aggregate([{$project:{bucketName:{$cond:{if:{$in:["aKey","$server.variables.key"]},then:{$arrayElemAt:["$server.variables.value",{$indexOfArray:["$server.variables.key","aKey"]}]},else:"NOT_FOUND"}}}}])