top_metrics is a metric aggregation that does not support sub aggregations. Currently if you add sub aggregation they will be silently ignored. Instead it should throw an error reporting it to the user. It is very easy to reproduce by using the example in the docs:

```
POST /test/_bulk?refresh
{"index": {}}
{"s": 1, "m": 3.1415}
{"index": {}}
{"s": 2, "m": 1.0}
{"index": {}}
{"s": 3, "m": 2.71828}

POST /test/_search?filter_path=aggregations
{
  "aggs": {
    "tm": {
      "top_metrics": {
        "metrics": {"field": "m"},
        "sort": {"s": "desc"}
      },
      "aggs": {
        "min": {
          "min": {
            "field": "m"
          }
        }
      }
    }
  }
}
```
x-pack/plugin/analytics/src/main/java/org/elasticsearch/xpack/analytics/topmetrics/TopMetricsAggregationBuilder.java
PARSER.declareInt(optionalConstructorArg(), SIZE_FIELD);
        ContextParser<Void, MultiValuesSourceFieldConfig.Builder> metricParser = MultiValuesSourceFieldConfig.parserBuilder(
            true,
            false,
            false,
            false,
            false
        );
        PARSER.declareObjectArray(constructorArg(), (p, n) -> metricParser.parse(p, null).build(), METRIC_FIELD);
    }
    private final List<SortBuilder<?>> sortBuilders;
    private final int size;
    private final List<MultiValuesSourceFieldConfig> metricFields;
    
    /**
     * Build a {@code top_metrics} aggregation request.
     */
    public TopMetricsAggregationBuilder(
        String name,
        List<SortBuilder<?>> sortBuilders,
        int size,
        List<MultiValuesSourceFieldConfig> metricFields
    ) {
        super(name);
        if (sortBuilders.size() != 1) {
            throw new IllegalArgumentException("[sort] must contain exactly one sort");
        }
        this.sortBuilders = sortBuilders;
        this.size = size;
        this.metricFields = metricFields;