### Elasticsearch Version

8.7.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

all

### Problem Description

Downsampling of a time series index without any metric field fails with a validation error:

```
Index <index> does not contain any metric fields;
```

Metric fields are not required in a time series index, unlike dimension fields. For this reason failing downsampling is not consistent with the idea of allowing time series indices without metrics.

### Steps to Reproduce

Create a time series index with at least one dimension field, without any metric field and run a downsample operation (through ILM or through the API endpoint directly).

### Logs (if relevant)

_No response_
x-pack/plugin/rollup/src/test/java/org/elasticsearch/xpack/downsample/DownsampleActionSingleNodeTests.java
import org.elasticsearch.search.aggregations.metrics.InternalTopHits;
import org.elasticsearch.search.aggregations.metrics.Max;
import org.elasticsearch.search.aggregations.metrics.MaxAggregationBuilder;
import org.elasticsearch.search.aggregations.metrics.MinAggregationBuilder;
import org.elasticsearch.search.aggregations.metrics.SumAggregationBuilder;
import org.elasticsearch.search.aggregations.metrics.TopHitsAggregationBuilder;
import org.elasticsearch.search.aggregations.metrics.ValueCountAggregationBuilder;
import org.elasticsearch.search.sort.SortBuilders;
import org.elasticsearch.search.sort.SortOrder;
import org.elasticsearch.tasks.TaskCancelHelper;
import org.elasticsearch.tasks.TaskCancelledException;
import org.elasticsearch.tasks.TaskId;
import org.elasticsearch.test.ESSingleNodeTestCase;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xcontent.XContentFactory;
import org.elasticsearch.xpack.aggregatemetric.AggregateMetricMapperPlugin;
import org.elasticsearch.xpack.core.LocalStateCompositeXPackPlugin;
import org.elasticsearch.xpack.core.downsample.DownsampleAction;
import org.elasticsearch.xpack.core.downsample.DownsampleConfig;
import org.elasticsearch.xpack.core.ilm.LifecycleSettings;
import org.elasticsearch.xpack.core.ilm.RolloverAction;
import org.elasticsearch.xpack.core.rollup.ConfigTestHelpers;
import org.elasticsearch.xpack.core.rollup.action.RollupShardTask;
import org.elasticsearch.xpack.ilm.IndexLifecycle;
import org.elasticsearch.xpack.rollup.Rollup;
import org.junit.Before;
import java.io.IOException;
import java.time.Instant;
import java.time.ZoneId;
import java.time.ZoneOffset;