[{"_id":"123456789","name":"item name 1","variants":[{"size":"150 ml","filters":[{"name":"filter 1","value":"value 1"},{"name":"filter 2","value":"value 2"}]}]},{"_id":"987654321","name":"item name 1","variants":[{"size":"200 ml","filters":[{"name":"filter 1","value":"value 2"},{"name":"filter 2","value":"value 1"}]}]}]

db.collection.aggregate([{$unwind:"$variants"},{$unwind:"$variants.filters"},{$group:{_id:"$variants.filters",count:{$sum:1}}},{$addFields:{"name":"$_id.name","value":"$_id.value"}},{$project:{_id:0}}])