### 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
IndicesService indicesService,
        ClusterService clusterService,
        TransportService transportService,
        ThreadPool threadPool,
        MetadataCreateIndexService metadataCreateIndexService,
        ActionFilters actionFilters,
        IndexNameExpressionResolver indexNameExpressionResolver,
        IndexScopedSettings indexScopedSettings
    ) {
        super(
            DownsampleAction.NAME,
            transportService,
            clusterService,
            threadPool,
            actionFilters,
            DownsampleAction.Request::new,
            indexNameExpressionResolver,
            ThreadPool.Names.SAME
        );
        this.client = new OriginSettingClient(client, ClientHelper.ROLLUP_ORIGIN);
        this.indicesService = indicesService;
        this.clusterService = clusterService;
        this.metadataCreateIndexService = metadataCreateIndexService;
        this.indexScopedSettings = indexScopedSettings;
        this.threadContext = threadPool.getThreadContext();
        this.taskQueue = clusterService.createTaskQueue("rollup", Priority.URGENT, STATE_UPDATE_TASK_EXECUTOR);
    }
    @Override
    protected void masterOperation(
        Task task,