### 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 java.time.ZonedDateTime;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Optional;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;
import static java.util.Collections.emptyMap;
import static org.elasticsearch.index.mapper.TimeSeriesParams.TIME_SERIES_METRIC_PARAM;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount;
import static org.elasticsearch.xcontent.XContentFactory.jsonBuilder;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
public class DownsampleActionSingleNodeTests extends ESSingleNodeTestCase {
    private static final DateFormatter DATE_FORMATTER = DateFormatter.forPattern("yyyy-MM-dd'T'HH:mm:ss.SSSXXX");
    public static final String FIELD_TIMESTAMP = "@timestamp";
    public static final String FIELD_DIMENSION_1 = "dimension_kw";
    public static final String FIELD_DIMENSION_2 = "dimension_long";
    public static final String FIELD_NUMERIC_1 = "numeric_1";
    public static final String FIELD_NUMERIC_2 = "numeric_2";
    public static final String FIELD_AGG_METRIC = "agg_metric_1";
    public static final String FIELD_METRIC_LABEL_DOUBLE = "metric_label_double";
    public static final String FIELD_LABEL_DOUBLE = "label_double";
    public static final String FIELD_LABEL_INTEGER = "label_integer";