Database
[{server:"strong",events:[{eventType:"log",createdAt:"2022-01-23T10:26:11.214Z",visitorInfo:{visitorId:"JohnID"}},{eventType:"log",createdAt:"2022-01-30T10:26:11.214Z",visitorInfo:{visitorId:"JohnID"}},{eventType:"system",createdAt:"2022-01-29T10:26:11.214Z",visitorInfo:{visitorId:"JennyID"}},{eventType:"system",createdAt:"2022-01-31T10:16:11.214Z",visitorInfo:{visitorId:"JennyID"}}]}]
Query
db.collection.aggregate([{$match:{server:"strong"}},{$project:{total:{$let:{vars:{events:{$filter:{input:"$events",cond:{$eq:["$$this.eventType","log"]}}}},in:{visitor:"$$events.visitorInfo.visitorId",uniquevisitor:{$setUnion:"$$events.visitorInfo.visitorId"}}}}}}])