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
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.search.aggregations.AbstractAggregationBuilder;
import org.elasticsearch.search.aggregations.AggregationBuilder;
import org.elasticsearch.search.aggregations.AggregatorFactories;
import org.elasticsearch.search.aggregations.AggregatorFactories.Builder;
import org.elasticsearch.search.aggregations.AggregatorFactory;
import org.elasticsearch.search.aggregations.support.AggregationContext;
import org.elasticsearch.search.aggregations.support.CoreValuesSourceType;
import org.elasticsearch.search.aggregations.support.MultiValuesSourceFieldConfig;
import org.elasticsearch.search.aggregations.support.TimeSeriesValuesSourceType;
import org.elasticsearch.search.aggregations.support.ValuesSourceRegistry;
import org.elasticsearch.search.aggregations.support.ValuesSourceRegistry.RegistryKey;
import org.elasticsearch.search.sort.SortBuilder;
import org.elasticsearch.xcontent.ConstructingObjectParser;
import org.elasticsearch.xcontent.ContextParser;
import org.elasticsearch.xcontent.ObjectParser;
import org.elasticsearch.xcontent.ParseField;
import org.elasticsearch.xcontent.XContentBuilder;
import java.io.IOException;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;
import static org.elasticsearch.search.builder.SearchSourceBuilder.SIZE_FIELD;
import static org.elasticsearch.search.builder.SearchSourceBuilder.SORT_FIELD;
import static org.elasticsearch.xcontent.ConstructingObjectParser.constructorArg;
import static org.elasticsearch.xcontent.ConstructingObjectParser.optionalConstructorArg;
public class TopMetricsAggregationBuilder extends AbstractAggregationBuilder<TopMetricsAggregationBuilder> {