### 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
DownsampleConfig config = new DownsampleConfig(randomInterval());
        SourceSupplier sourceSupplier = () -> XContentFactory.jsonBuilder()
            .startObject()
            .field(FIELD_TIMESTAMP, randomDateForInterval(config.getInterval()))
            .field(FIELD_DIMENSION_1, randomAlphaOfLength(1))
            .field(FIELD_NUMERIC_1, randomDouble())
            .endObject();
        bulkIndex(sourceSupplier);
        prepareSourceIndex(sourceIndex);
        IndicesService indexServices = getInstanceFromNode(IndicesService.class);
        Index srcIndex = resolveIndex(sourceIndex);
        IndexService indexService = indexServices.indexServiceSafe(srcIndex);
        int shardNum = randomIntBetween(0, numOfShards - 1);
        IndexShard shard = indexService.getShard(shardNum);
        RollupShardTask task = new RollupShardTask(
            randomLong(),
            "rollup",
            "action",
            TaskId.EMPTY_TASK_ID,
            rollupIndex,
            config,
            emptyMap(),
            shard.shardId()
        );
        
        RollupShardIndexer indexer = new RollupShardIndexer(
            task,
            client(),
            indexService,