### 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
DownsampleAction.Request request,
        ActionListener<AcknowledgedResponse> listener
    ) {
        /*
         * When creating the rollup index, we copy the index.number_of_shards from source index,
         * and we set the index.number_of_replicas to 0, to avoid replicating the index being built.
         * Also, we set the index.refresh_interval to -1.
         * We will set the correct number of replicas and refresh the index later.
         *
         * We should note that there is a risk of losing a node during the rollup process. In this
         * case rollup will fail.
         */
        Settings.Builder builder = Settings.builder()
            .put(IndexMetadata.SETTING_INDEX_HIDDEN, true)
            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, sourceIndexMetadata.getNumberOfShards())
            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
            .put(IndexSettings.INDEX_REFRESH_INTERVAL_SETTING.getKey(), "-1")
            .put(IndexMetadata.INDEX_DOWNSAMPLE_STATUS.getKey(), IndexMetadata.DownsampleTaskStatus.STARTED);
        if (sourceIndexMetadata.getSettings().hasValue(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())) {
            builder.put(
                MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey(),
                sourceIndexMetadata.getSettings().get(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())
            );
        }
        CreateIndexClusterStateUpdateRequest createIndexClusterStateUpdateRequest = new CreateIndexClusterStateUpdateRequest(
            "rollup",
            rollupIndexName,
            rollupIndexName
        ).settings(builder.build()).mappings(mapping);
        var delegate = new AllocationActionListener<>(listener, threadPool.getThreadContext());