### 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
builder.endObject(); 
        builder.endObject(); 
        final CompressedXContent rollupDiffXContent = CompressedXContent.fromJSON(
            XContentHelper.convertToJson(BytesReference.bytes(builder), false, XContentType.JSON)
        );
        return mapperService.merge(MapperService.SINGLE_MAPPING_NAME, rollupDiffXContent, MapperService.MergeReason.INDEX_TEMPLATE)
            .mappingSource()
            .uncompressed()
            .utf8ToString();
    }
    private static void addMetricFields(
        final TimeseriesFieldTypeHelper helper,
        final Map<String, Object> sourceIndexMappings,
        final XContentBuilder builder
    ) {
        MappingVisitor.visitMapping(sourceIndexMappings, (field, mapping) -> {
            if (helper.isTimeSeriesMetric(field, mapping)) {
                try {
                    addMetricFieldMapping(builder, field, mapping);
                } catch (IOException e) {
                    throw new ElasticsearchException("Error while adding metric for field [" + field + "]");
                }
            }
        });
    }
    private static void addTimestampField(
        final DownsampleConfig config,
        Map<String, Object> sourceIndexMappings,
        final XContentBuilder builder
    ) throws IOException {