### 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.cluster.metadata.Template;
import org.elasticsearch.common.compress.CompressedXContent;
import org.elasticsearch.common.document.DocumentField;
import org.elasticsearch.common.network.NetworkAddress;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.time.DateFormatter;
import org.elasticsearch.common.unit.ByteSizeValue;
import org.elasticsearch.datastreams.DataStreamsPlugin;
import org.elasticsearch.index.Index;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexNotFoundException;
import org.elasticsearch.index.IndexService;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.engine.VersionConflictEngineException;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.index.mapper.MapperService;
import org.elasticsearch.index.mapper.TimeSeriesIdFieldMapper;
import org.elasticsearch.index.mapper.TimeSeriesParams;
import org.elasticsearch.index.shard.IndexShard;
import org.elasticsearch.indices.IndicesService;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.search.SearchHit;
import org.elasticsearch.search.aggregations.Aggregation;
import org.elasticsearch.search.aggregations.AggregationBuilder;
import org.elasticsearch.search.aggregations.Aggregations;
import org.elasticsearch.search.aggregations.bucket.histogram.DateHistogramAggregationBuilder;
import org.elasticsearch.search.aggregations.bucket.histogram.DateHistogramInterval;
import org.elasticsearch.search.aggregations.bucket.histogram.InternalDateHistogram;
import org.elasticsearch.search.aggregations.bucket.terms.StringTerms;
import org.elasticsearch.search.aggregations.bucket.terms.TermsAggregationBuilder;