### 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/test/java/org/elasticsearch/xpack/downsample/DownsampleActionSingleNodeTests.java
new MinAggregationBuilder(fieldName + "_" + supportedAggregation).field(fieldName)
                    );
                    case "max" -> dateHistogramAggregation.subAggregation(
                        new MaxAggregationBuilder(fieldName + "_" + supportedAggregation).field(fieldName)
                    );
                    case "last_value" -> dateHistogramAggregation.subAggregation(
                        new TopHitsAggregationBuilder(fieldName + "_" + supportedAggregation).sort(
                            SortBuilders.fieldSort(timestampField).order(SortOrder.DESC)
                        ).size(1).fetchField(fieldName)
                    );
                    case "sum" -> dateHistogramAggregation.subAggregation(
                        new SumAggregationBuilder(fieldName + "_" + supportedAggregation).field(fieldName)
                    );
                    case "value_count" -> dateHistogramAggregation.subAggregation(
                        new ValueCountAggregationBuilder(fieldName + "_" + supportedAggregation).field(fieldName)
                    );
                    default -> throw new IllegalArgumentException("Unsupported metric type [" + supportedAggregation + "]");
                }
            }
        });
        labels.forEach((fieldName, type) -> {
            dateHistogramAggregation.subAggregation(
                new TopHitsAggregationBuilder(fieldName + "_last_value").sort(SortBuilders.fieldSort(timestampField).order(SortOrder.DESC))
                    .size(1)
                    .fetchField(fieldName)
            );
        });
        return tsidAggregation;
    }
    @FunctionalInterface