### 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
final String timestampField = config.getTimestampField();
        final String dateIntervalType = config.getIntervalType();
        final String dateInterval = config.getInterval().toString();
        final String timezone = config.getTimeZone();
        builder.startObject(timestampField);
        MappingVisitor.visitMapping(sourceIndexMappings, (field, mapping) -> {
            try {
                if (timestampField.equals(field)) {
                    final String timestampType = String.valueOf(mapping.get("type"));
                    builder.field("type", timestampType != null ? timestampType : DateFieldMapper.CONTENT_TYPE);
                    if (mapping.get("format") != null) {
                        builder.field("format", mapping.get("format"));
                    }
                }
            } catch (IOException e) {
                throw new ElasticsearchException("Unable to create timestamp field mapping for field [" + timestampField + "]", e);
            }
        });
        builder.startObject("meta")
            .field(dateIntervalType, dateInterval)
            .field(DownsampleConfig.TIME_ZONE, timezone)
            .endObject()
            .endObject();
    }
    private static void addMetricFieldMapping(final XContentBuilder builder, final String field, final Map<String, ?> fieldProperties)
        throws IOException {
        final TimeSeriesParams.MetricType metricType = TimeSeriesParams.MetricType.fromString(
            fieldProperties.get(TIME_SERIES_METRIC_PARAM).toString()
        );
        if (metricType == TimeSeriesParams.MetricType.COUNTER) {