Support multiple metrics in top_metrics agg (backport of #52965)

[nik9000]

This adds support for returning multiple metrics to the top_metrics
agg. It looks like:

POST /test/_search?filter_path=aggregations
{
  "aggs": {
    "tm": {
      "top_metrics": {
        "metrics": [
          {"field": "v"},
          {"field": "m"}
        ],
        "sort": {"s": "desc"}
      }
    }
  }
}