### 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
prepareSourceIndex(sourceIndex);
        rollup(sourceIndex, rollupIndex, config);
        assertRollupIndex(sourceIndex, rollupIndex, config);
    }
    public void testCannotRollupIndexWithNoMetrics() {
        
        String sourceIndex = "no-metrics-idx-" + randomAlphaOfLength(5).toLowerCase(Locale.ROOT);
        client().admin()
            .indices()
            .prepareCreate(sourceIndex)
            .setSettings(
                Settings.builder()
                    .put("index.number_of_shards", numOfShards)
                    .put("index.number_of_replicas", numOfReplicas)
                    .put(IndexSettings.MODE.getKey(), IndexMode.TIME_SERIES)
                    .putList(IndexMetadata.INDEX_ROUTING_PATH.getKey(), List.of(FIELD_DIMENSION_1))
                    .put(IndexSettings.TIME_SERIES_START_TIME.getKey(), Instant.ofEpochMilli(startTime).toString())
                    .put(IndexSettings.TIME_SERIES_END_TIME.getKey(), "2106-01-08T23:40:53.384Z")
                    .build()
            )
            .setMapping(
                FIELD_TIMESTAMP,
                "type=date",
                FIELD_DIMENSION_1,
                "type=keyword,time_series_dimension=true",
                FIELD_DIMENSION_2,
                "type=long,time_series_dimension=true"
            )
            .get();
        DownsampleConfig config = new DownsampleConfig(randomInterval());