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
public static final String NAME = "top_metrics";
    public static final ParseField METRIC_FIELD = new ParseField("metrics");
    static final RegistryKey<TopMetricsAggregator.MetricValuesSupplier> REGISTRY_KEY = new RegistryKey<>(
        TopMetricsAggregationBuilder.NAME,
        TopMetricsAggregator.MetricValuesSupplier.class
    );
    public static void registerAggregators(ValuesSourceRegistry.Builder registry) {
        registry.registerUsage(NAME);
        registry.register(
            REGISTRY_KEY,
            List.of(CoreValuesSourceType.NUMERIC, TimeSeriesValuesSourceType.COUNTER),
            TopMetricsAggregator::buildNumericMetricValues,
            false
        );
        registry.register(
            REGISTRY_KEY,
            List.of(CoreValuesSourceType.BOOLEAN, CoreValuesSourceType.DATE),
            TopMetricsAggregator.LongMetricValues::new,
            false
        );
        registry.register(
            REGISTRY_KEY,
            List.of(CoreValuesSourceType.KEYWORD, CoreValuesSourceType.IP),