public function count(string $fieldName) : Stage\Count
    {
        $stage = new Stage\Count($this, $fieldName);
        $this->addStage($stage);

        return $stage;
    }
public function count(string $fieldName) : Stage\Count
    {
        $stage = new Stage\Count($this, $fieldName);
        $this->addStage($stage);

        return $stage;
    }
public function count ( string $ fieldName ) : Stage \ Count { $ stage = new Stage \ Count ( $ this , $ fieldName ) ; $ this -> addStage ( $ stage ) ; return $ stage ; }
Returns a document that contains a count of the number of documents input
to the stage.

@see https://docs.mongodb.com/manual/reference/operator/aggregation/count/
Returns a document that contains a count of the number of documents input to the stage .