Transform support for top_metrics

#51155 is adding a `top_metrics` aggregation which is similar to `top_hits` but trades flexibility for memory consumption and a little speed. This tracks transform's usage of it, as requested by [this](https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/transform/src/test/java/org/elasticsearch/xpack/transform/transforms/pivot/AggregationsTests.java#L94-L101) exception message.