### 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
new ElasticsearchException(
                    "Rollup requires setting ["
                        + IndexSettings.MODE.getKey()
                        + "="
                        + IndexMode.TIME_SERIES
                        + "] for index ["
                        + sourceIndexName
                        + "]"
                )
            );
            return;
        }
        
        if (state.blocks().indexBlocked(ClusterBlockLevel.WRITE, sourceIndexName) == false) {
            listener.onFailure(
                new ElasticsearchException(
                    "Rollup requires setting [" + IndexMetadata.SETTING_BLOCKS_WRITE + " = true] for index [" + sourceIndexName + "]"
                )
            );
            return;
        }
        final String rollupIndexName = request.getTargetIndex();
        
        MetadataCreateIndexService.validateIndexName(rollupIndexName, state);