### 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
);
        assertNotNull(sourceSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey()));
        assertNotNull(rollupSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey()));
        assertEquals(
            sourceSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey()),
            rollupSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey())
        );
        assertNotNull(sourceSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey()));
        assertNotNull(rollupSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey()));
        assertEquals(
            sourceSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey()),
            rollupSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey())
        );
        assertNotNull(sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS));
        assertNotNull(rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS));
        assertEquals(
            sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS),
            rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS)
        );
        assertNotNull(sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS));
        assertNotNull(rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS));
        assertEquals(
            sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS),
            rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS)
        );
        assertEquals("true", rollupSettings.get(IndexMetadata.SETTING_BLOCKS_WRITE));
        assertEquals(sourceSettings.get(IndexMetadata.SETTING_INDEX_HIDDEN), rollupSettings.get(IndexMetadata.SETTING_INDEX_HIDDEN));
        if (sourceSettings.keySet().contains(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())) {
            assertNotNull(indexSettingsResp.getSetting(rollupIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()));
        }