In APM, we use a top_hits aggregation in some places to get certain values for the UI. E.g., if we are aggregation over service.name, we also need agent.name or each value of service.name. A top_hits aggregation can be costly, especially when there are many buckets.

Recently top_metrics was introduced as a lighter, faster alternative to top_hits. However, it does not support keywords (yet).

We would benefit from having top_metrics support keywords or a different aggregation that does the same, but for keywords.