### 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
} else {
                    listener.onFailure(new ElasticsearchException("Failed to create rollup index [" + rollupIndexName + "]"));
                }
            }, listener::onFailure));
        }, listener::onFailure));
    }
    @Override
    protected ClusterBlockException checkBlock(DownsampleAction.Request request, ClusterState state) {
        return state.blocks().globalBlockedException(ClusterBlockLevel.METADATA_WRITE);
    }
    /**
     * This method creates the mapping for the rollup index, based on the
     * mapping (dimensions and metrics) from the source index, as well as the
     * rollup configuration.
     *
     * @param config the rollup configuration
     * @param sourceIndexMappings a map with the source index mapping
     * @return the mapping of the rollup index
     */
    public static String createRollupIndexMapping(
        final TimeseriesFieldTypeHelper helper,
        final DownsampleConfig config,
        final MapperService mapperService,
        final Map<String, Object> sourceIndexMappings
    ) throws IOException {
        final XContentBuilder builder = XContentFactory.jsonBuilder().startObject();
        addDynamicTemplates(builder);
        builder.startObject("properties");
        addTimestampField(config, sourceIndexMappings, builder);
        addMetricFields(helper, sourceIndexMappings, builder);