### 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.startObject(field).field("type", fieldProperties.get("type")).field(TIME_SERIES_METRIC_PARAM, metricType).endObject();
        } else {
            final List<String> supportedAggs = List.of(metricType.supportedAggs());
            
            final String defaultMetric = supportedAggs.contains("max") ? "max" : supportedAggs.get(0);
            builder.startObject(field)
                .field("type", AggregateDoubleMetricFieldMapper.CONTENT_TYPE)
                .stringListField(AggregateDoubleMetricFieldMapper.Names.METRICS, supportedAggs)
                .field(AggregateDoubleMetricFieldMapper.Names.DEFAULT_METRIC, defaultMetric)
                .field(TIME_SERIES_METRIC_PARAM, metricType)
                .endObject();
        }
    }
    private static void validateDownsamplingInterval(MapperService mapperService, DownsampleConfig config) {
        MappedFieldType timestampFieldType = mapperService.fieldType(config.getTimestampField());
        assert timestampFieldType != null : "Cannot find timestamp field [" + config.getTimestampField() + "] in the mapping";
        ActionRequestValidationException e = new ActionRequestValidationException();
        Map<String, String> meta = timestampFieldType.meta();
        if (meta.isEmpty() == false) {
            String interval = meta.get(config.getIntervalType());
            if (interval != null) {
                DateHistogramInterval sourceIndexInterval = new DateHistogramInterval(interval);
                DateHistogramInterval targetIndexInterval = config.getInterval();
                long sourceMillis = sourceIndexInterval.estimateMillis();
                long targetMillis = targetIndexInterval.estimateMillis();
                if (sourceMillis >= targetMillis) {
                    
                    e.addValidationError(