### 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/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleAction.java
* If the source index is a rollup index, we will add the name and UUID
         * of the first index that we initially rolled up.
         */
        if (IndexMetadata.INDEX_DOWNSAMPLE_SOURCE_UUID.exists(sourceIndexMetadata.getSettings()) == false
            || IndexMetadata.INDEX_DOWNSAMPLE_SOURCE_NAME.exists(sourceIndexMetadata.getSettings()) == false) {
            Index sourceIndex = sourceIndexMetadata.getIndex();
            targetSettings.put(IndexMetadata.INDEX_DOWNSAMPLE_SOURCE_NAME.getKey(), sourceIndex.getName())
                .put(IndexMetadata.INDEX_DOWNSAMPLE_SOURCE_UUID.getKey(), sourceIndex.getUUID());
        }
        return IndexMetadata.builder(rollupIndexMetadata).settings(targetSettings);
    }
    /**
     * Configure the dynamic templates to always map strings to the keyword field type.
     */
    private static void addDynamicTemplates(final XContentBuilder builder) throws IOException {
        builder.startArray("dynamic_templates")
            .startObject()
            .startObject("strings")
            .field("match_mapping_type", "string")
            .startObject("mapping")
            .field("type", "keyword")
            .endObject()
            .endObject()
            .endObject()
            .endArray();
    }
    private void createRollupIndex(
        String rollupIndexName,
        IndexMetadata sourceIndexMetadata,
        String mapping,