### 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
startTime = randomLongBetween(946769284000L, 1607470084000L); 
        docCount = randomIntBetween(10, 9000);
        numOfShards = randomIntBetween(1, 4);
        numOfReplicas = randomIntBetween(0, 3);
        
        dimensionValues = new ArrayList<>(MAX_DIM_VALUES);
        for (int j = 0; j < randomIntBetween(1, MAX_DIM_VALUES); j++) {
            dimensionValues.add(randomAlphaOfLength(6));
        }
        /**
         * NOTE: here we map each numeric label field also as a (counter) metric.
         * This is done for testing purposes. There is no easy way to test
         * that labels are collected using the last value. The idea is to
         * check that the value of the label (last value) matches the value
         * of the corresponding metric which uses a last_value metric type.
         */
        Settings.Builder settings = Settings.builder()
            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, numOfShards)
            .put(IndexMetadata.SETTING_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(),
                DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.formatMillis(Instant.ofEpochMilli(startTime).toEpochMilli())
            )
            .put(IndexSettings.TIME_SERIES_END_TIME.getKey(), "2106-01-08T23:40:53.384Z");
        if (randomBoolean()) {
            settings.put(IndexMetadata.SETTING_INDEX_HIDDEN, randomBoolean());
        }
        XContentBuilder mapping = jsonBuilder().startObject().startObject("_doc").startObject("properties");