### 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
private static final Logger logger = LogManager.getLogger(TransportDownsampleAction.class);
    private final Client client;
    private final IndicesService indicesService;
    private final ClusterService clusterService;
    private final MasterServiceTaskQueue<RollupClusterStateUpdateTask> taskQueue;
    private final MetadataCreateIndexService metadataCreateIndexService;
    private final IndexScopedSettings indexScopedSettings;
    private final ThreadContext threadContext;
    /**
     * This is the cluster state task executor for cluster state update actions.
     */
    private static final SimpleBatchedExecutor<RollupClusterStateUpdateTask, Void> STATE_UPDATE_TASK_EXECUTOR =
        new SimpleBatchedExecutor<>() {
            @Override
            public Tuple<ClusterState, Void> executeTask(RollupClusterStateUpdateTask task, ClusterState clusterState) throws Exception {
                return Tuple.tuple(task.execute(clusterState), null);
            }
            @Override
            public void taskSucceeded(RollupClusterStateUpdateTask task, Void unused) {
                task.listener.onResponse(AcknowledgedResponse.TRUE);
            }
        };
    @Inject
    public TransportDownsampleAction(
        Client client,