### 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
);
                                                }
                                            },
                                                e -> deleteRollupIndex(
                                                    sourceIndexName,
                                                    rollupIndexName,
                                                    parentTask,
                                                    listener,
                                                    new ElasticsearchException(
                                                        "Failed to publish new cluster state with rollup metadata",
                                                        e
                                                    )
                                                )
                                            ));
                                        } else {
                                            deleteRollupIndex(
                                                sourceIndexName,
                                                rollupIndexName,
                                                parentTask,
                                                listener,
                                                new ElasticsearchException("Failed to refresh rollup index [" + rollupIndexName + "]")
                                            );
                                        }
                                    },
                                        e -> deleteRollupIndex(
                                            sourceIndexName,
                                            rollupIndexName,
                                            parentTask,
                                            listener,
                                            new ElasticsearchException("Failed to refresh rollup index [" + rollupIndexName + "]", e)