### 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
)
                                    ));
                                } else {
                                    deleteRollupIndex(
                                        sourceIndexName,
                                        rollupIndexName,
                                        parentTask,
                                        listener,
                                        new ElasticsearchException("Unable to update settings of rollup index [" + rollupIndexName + "]")
                                    );
                                }
                            },
                                e -> deleteRollupIndex(
                                    sourceIndexName,
                                    rollupIndexName,
                                    parentTask,
                                    listener,
                                    new ElasticsearchException("Unable to update settings of rollup index [" + rollupIndexName + "]", e)
                                )
                            ));
                        } else {
                            deleteRollupIndex(
                                sourceIndexName,
                                rollupIndexName,
                                parentTask,
                                listener,
                                new ElasticsearchException("Unable to index into rollup index [" + rollupIndexName + "]")
                            );
                        }
                    }, e -> deleteRollupIndex(sourceIndexName, rollupIndexName, parentTask, listener, e)));