[{_id:ObjectId("5ed4d2ed8a98d217c467c040"),name:"Name1",description:"Desc1",tags:["Tag1","Tag3"]},{_id:ObjectId("5ed4d2ed8a98dd173467c040"),name:"Name2",description:"Desc2",tags:["Tag1","Tag3"]},{_id:ObjectId("5ed4d2ed8a98dd17c367c040"),name:"Name3",description:"Desc3",tags:["Tag1","Tag4"]},{_id:ObjectId("5ed4d2ed8a98dd17c467c140"),name:"Name4",description:"Desc4",tags:["Tag3","Tag4"]},{_id:ObjectId("5ed4d2ed8a98dd17c467c040"),name:"Name5",description:"Desc6",tags:["Tag1","Tag2"]}]

db.collection.aggregate([{"$unwind":"$tags"},{"$group":{"_id":"$tags","count":{"$sum":1}}},{"$sort":{"count":-1}},{"$group":{"_id":null,"tags":{"$push":"$_id"}}},{"$unset":"_id"}])