db={"users":[{"_id":{"$oid":"5f7e92b88dc8f64cb4e6c2c0"},"login":"mojombo","id":1,},{"_id":{"$oid":"5f7e92b98dc8f64cb4e6c2ff"},"login":"defunkt","id":2,},{"_id":{"$oid":"5f7e92bb8dc8f64cb4e6c300"},"login":"pjhyett","id":3,}],"counters":[{"_id":{"$oid":"5f8e5bde9054ba2477dc2c57"},"repoName":"ale","repoNumber":171780764,"userName":"mojombo","userNumber":1,"viewDate":"2020-10-20","count":6,},{"_id":{"$oid":"5f8e5bde9154ba2477dc2c57"},"repoName":"ale2","repoNumber":171780765,"userName":"mojombo","userNumber":1,"viewDate":"2020-10-20","count":1,},{"_id":{"$oid":"5f8e5bde9154ba2476dc2c57"},"repoName":"ale3","repoNumber":171210765,"userName":"pjhyett","userNumber":3,"viewDate":"2020-10-20","count":1,},{"_id":{"$oid":"5f8e53de9154ba2476dc2c57"},"repoName":"ale1341","repoNumber":17124765,"userName":"pjhyett","userNumber":3,"viewDate":"2020-10-20","count":1,},{"_id":{"$oid":"5f8e52de9154ba2477dc2c57"},"repoName":"ale4","repoNumber":173780765,"userName":"defunkt","userNumber":2,"viewDate":"2020-10-20","count":4,}]}

db.users.aggregate([{$lookup:{from:"counters",let:{id:"$id",login:"$login"},pipeline:[{$match:{$expr:{$eq:["$userName","$$login"],$eq:["$userNumber","$$id"]}}}],as:"count"}},{$addFields:{count:{$reduce:{input:"$count",initialValue:0,in:{$add:["$$value","$$this.count"]}}}}},{$skip:0},{$limit:1000}])