### 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/test/java/org/elasticsearch/xpack/downsample/DownsampleActionSingleNodeTests.java
public interface SourceSupplier {
        XContentBuilder get() throws IOException;
    }
    private String createDataStream() throws Exception {
        String dataStreamName = randomAlphaOfLength(10).toLowerCase(Locale.getDefault());
        Template indexTemplate = new Template(
            Settings.builder()
                .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, numOfShards)
                .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, numOfReplicas)
                .put(IndexSettings.MODE.getKey(), IndexMode.TIME_SERIES)
                .putList(IndexMetadata.INDEX_ROUTING_PATH.getKey(), List.of(FIELD_DIMENSION_1))
                .build(),
            new CompressedXContent("""
                {
                    "properties": {
                        "@timestamp" : {
                            "type": "date"
                        },
                        "dimension_kw": {
                            "type": "keyword",
                            "time_series_dimension": true
                        },
                        "dimension_long": {
                            "type": "long",
                            "time_series_dimension": true
                        },
                        "numeric_1": {