Documents counts for this aggregation are going to be the same as the document counts as the parent aggregation since there is no filtering performed. So maybe we shouldn't implement `SingleBucketAggregator` here since it forces to count documents.

This aggregation is similar to the `top_hits` aggregation in the sense that it doesn't do filtering, and shouldn't support sub-aggs so I was thinking that we might want to introduce a new `LeafAggregator` base class that would extend `Aggregator`, would neither support sub aggs nor count documents, and would in turn be extended by `MetricsAggregator`?