Rollup : add "count document" aggregation metric

[pmarjou22]

I'm currently using Rollup on 1.12.0, and I have to create a dummy field on all my documents to be able to count the number of documents from source index.

It would be nice to be able to add this aggregation directly from RollUp configuration without this workaround

Current Workaound with event.count set to 1 on all documents

Source Index
POST sourceindex/_doc/
{
"event.name" : "event1",
"event.category" : "category1",
"event.count" : 1,
"date": "2015-01-01T12:10:30Z"
}

POST sourceindex/_doc
{
"event.name" : "event2",
"event.category" : "category1",
"event.count" : 1,
"date": "2015-01-01T12:10:30Z"
}

POST sourceindex/_doc
{
"event.name" : "event3",
"event.category" : "category2",
"event.count" : 1,
"date": "2015-01-01T12:10:30Z"
}

Target Index workaround : Sum on event.count