### 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
"Source index is a downsampled index. Downsampling interval ["
                            + targetIndexInterval
                            + "] must be greater than the source index interval ["
                            + sourceIndexInterval
                            + "]"
                    );
                } else if (targetMillis % sourceMillis != 0) {
                    
                    e.addValidationError(
                        "Source index is a downsampled index. Downsampling interval ["
                            + targetIndexInterval
                            + "] must be a multiple of the source index interval ["
                            + sourceIndexInterval
                            + "]"
                    );
                }
            }
            
            String sourceTimezone = meta.get(DownsampleConfig.TIME_ZONE);
            if (sourceTimezone != null && sourceTimezone.equals(config.getTimeZone()) == false) {
                e.addValidationError(
                    "Source index is a downsampled index. Downsampling timezone ["
                        + config.getTimeZone()
                        + "] cannot be different than the source index timezone ["
                        + sourceTimezone
                        + "]"
                );
            }
            if (e.validationErrors().isEmpty() == false) {
                throw e;