### 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
}
        }
    }
    /**
     * Copy index settings from the source index to the rollup index. Settings that
     * have already been set in the rollup index will not be overridden.
     */
    private IndexMetadata.Builder copyIndexMetadata(IndexMetadata sourceIndexMetadata, IndexMetadata rollupIndexMetadata) {
        
        
        final Settings.Builder targetSettings = Settings.builder().put(rollupIndexMetadata.getSettings());
        for (final String key : sourceIndexMetadata.getSettings().keySet()) {
            final Setting<?> setting = indexScopedSettings.get(key);
            if (setting == null) {
                assert indexScopedSettings.isPrivateSetting(key) : "expected [" + key + "] to be private but it was not";
            } else if (setting.getProperties().contains(Setting.Property.NotCopyableOnResize)) {
                
                
                continue;
            }
            
            
            
            if (IndexMetadata.SETTING_BLOCKS_WRITE.equals(key) || targetSettings.keys().contains(key)) {
                continue;
            }
            targetSettings.copy(key, sourceIndexMetadata.getSettings());
        }
        /*
         * Add the source index name and UUID to the rollup index metadata.