[{"_id":ObjectId("5e6012626536339ffebef710"),"id":"5e5fb732a9422a001146509f","customPicklistValues":[{"id":"5e4e904f16ab94bff1a324a0","value":"Internal","fieldName":"Business Group","customPicklist":"109c7a1a9d00b664f2ee7827"},{"id":"5e4e904f16ab94bff1a324a4","value":"Slack","fieldName":"Application","customPicklist":"109c5a1a9d00b664f2ee7827"}]},{"_id":ObjectId("5e6012686536339ffebef789"),"id":"5e5fb732a9422a0011465091","customPicklistValues":[{"id":"5e4e904f16ab94bff1a324a0","value":"Internal","fieldName":"Business Group","customPicklist":"109c7a1a9d00b664f2ee7827"},{"id":"5e4e904f16ab94bff1a324a4","value":"Slack","fieldName":"Application","customPicklist":"109c5a1a9d00b664f2ee7827"}]}]

db.collection.aggregate([{$addFields:{customPicklistValues:{$arrayToObject:{$map:{input:"$customPicklistValues",in:{k:"$$this.fieldName",v:"$$this.value"}}}}}},{$replaceRoot:{newRoot:{$mergeObjects:["$customPicklistValues","$$ROOT"]}}},{$project:{customPicklistValues:0}}])