### 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
taskQueue.submitTask("create-rollup-index [" + rollupIndexName + "]", new RollupClusterStateUpdateTask(listener) {
            @Override
            public ClusterState execute(ClusterState currentState) throws Exception {
                return metadataCreateIndexService.applyCreateIndexRequest(
                    currentState,
                    createIndexClusterStateUpdateRequest,
                    true,
                    
                    (builder, rollupIndexMetadata) -> builder.put(copyIndexMetadata(sourceIndexMetadata, rollupIndexMetadata)),
                    delegate.reroute()
                );
            }
        }, request.masterNodeTimeout());
    }
    private void updateRollupMetadata(
        String rollupIndexName,
        DownsampleAction.Request request,
        ActionListener<AcknowledgedResponse> listener
    ) {
        
        taskQueue.submitTask("update-rollup-metadata [" + rollupIndexName + "]", new RollupClusterStateUpdateTask(listener) {
            @Override
            public ClusterState execute(ClusterState currentState) {
                Metadata metadata = currentState.metadata();
                Metadata.Builder metadataBuilder = Metadata.builder(metadata);
                Index rollupIndex = metadata.index(rollupIndexName).getIndex();
                IndexMetadata rollupIndexMetadata = metadata.index(rollupIndex);
                metadataBuilder.updateSettings(
                    Settings.builder()
                        .put(rollupIndexMetadata.getSettings())