### 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
mapping.startObject(FIELD_TIMESTAMP).field("type", "date").endObject();
        
        mapping.startObject(FIELD_DIMENSION_1).field("type", "keyword").field("time_series_dimension", true).endObject();
        mapping.startObject(FIELD_DIMENSION_2).field("type", "long").field("time_series_dimension", true).endObject();
        
        mapping.startObject(FIELD_NUMERIC_1).field("type", "long").field("time_series_metric", "gauge").endObject();
        mapping.startObject(FIELD_NUMERIC_2).field("type", "double").field("time_series_metric", "counter").endObject();
        mapping.startObject(FIELD_AGG_METRIC)
            .field("type", "aggregate_metric_double")
            .field("time_series_metric", "gauge")
            .array("metrics", new String[] { "min", "max", "sum", "value_count" })
            .field("default_metric", "value_count")
            .endObject();
        mapping.startObject(FIELD_METRIC_LABEL_DOUBLE)
            .field("type", "double") 
            .field("time_series_metric", "counter")
            .endObject();
        
        mapping.startObject(FIELD_LABEL_DOUBLE).field("type", "double").endObject();
        mapping.startObject(FIELD_LABEL_INTEGER).field("type", "integer").endObject();
        mapping.startObject(FIELD_LABEL_KEYWORD).field("type", "keyword").endObject();
        mapping.startObject(FIELD_LABEL_TEXT).field("type", "text").field("store", "true").endObject();
        mapping.startObject(FIELD_LABEL_BOOLEAN).field("type", "boolean").endObject();
        mapping.startObject(FIELD_LABEL_IPv4_ADDRESS).field("type", "ip").endObject();
        mapping.startObject(FIELD_LABEL_IPv6_ADDRESS).field("type", "ip").endObject();
        mapping.startObject(FIELD_LABEL_DATE).field("type", "date").field("format", "date_optional_time").endObject();
        mapping.startObject(FIELD_LABEL_KEYWORD_ARRAY).field("type", "keyword").endObject();
        mapping.startObject(FIELD_LABEL_DOUBLE_ARRAY).field("type", "double").endObject();
        mapping.startObject(FIELD_LABEL_AGG_METRIC)
            .field("type", "aggregate_metric_double")