MongoDB Aggregation: How to get total records count?

Since v.3.4 (i think) MongoDB has now a new aggregation pipeline operator named ‘facet‘ which in their own words:

Processes multiple aggregation pipelines within a single stage on the same set of input documents. Each sub-pipeline has its own field in the output document where its results are stored as an array of documents.

In this particular case, this means that one can do something like this:

$result = $collection->aggregate([
  { ...execute queries, group, sort... },
  { ...execute queries, group, sort... },
  { ...execute queries, group, sort... },
  {
    $facet: {
      paginatedResults: [{ $skip: skipPage }, { $limit: perPage }],
      totalCount: [
        {
          $count: 'count'
        }
      ]
    }
  }
]);

The result will be (with for ex 100 total results):

[
  {
    "paginatedResults":[{...},{...},{...}, ...],
    "totalCount":[{"count":100}]
  }
]

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