### 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
updateRollupMetadata(rollupIndexName, request, ActionListener.wrap(resp -> {
                                                if (resp.isAcknowledged()) {
                                                    
                                                    forceMergeIndex(
                                                        rollupIndexName,
                                                        parentTask,
                                                        ActionListener.wrap(
                                                            mergeIndexResp -> listener.onResponse(AcknowledgedResponse.TRUE),
                                                            e -> {
                                                                /*
                                                                 * At this point rollup has been created successfully even if
                                                                 * force-merge fails. So, we should not fail the rollup operation.
                                                                 */
                                                                logger.error(
                                                                    "Failed to force-merge rollup index [" + rollupIndexName + "]",
                                                                    e
                                                                );
                                                                listener.onResponse(AcknowledgedResponse.TRUE);
                                                            }
                                                        )
                                                    );
                                                } else {
                                                    deleteRollupIndex(
                                                        sourceIndexName,
                                                        rollupIndexName,
                                                        parentTask,
                                                        listener,
                                                        new ElasticsearchException(
                                                            "Failed to publish new cluster state with rollup metadata"
                                                        )