### 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
"type": "long",
                            "time_series_metric": "gauge"
                        },
                        "numeric_2": {
                            "type": "double",
                            "time_series_metric": "counter"
                        }
                    }
                }
                """),
            null
        );
        ComposableIndexTemplate template = new ComposableIndexTemplate(
            List.of(dataStreamName + "*"),
            indexTemplate,
            null,
            null,
            null,
            null,
            new ComposableIndexTemplate.DataStreamTemplate(false, false),
            null
        );
        PutComposableIndexTemplateAction.Request request = new PutComposableIndexTemplateAction.Request(dataStreamName + "_template")
            .indexTemplate(template);
        assertAcked(client().execute(PutComposableIndexTemplateAction.INSTANCE, request).actionGet());
        assertAcked(client().execute(CreateDataStreamAction.INSTANCE, new CreateDataStreamAction.Request(dataStreamName)).get());
        return dataStreamName;
    }
}