### 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/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleAction.java
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.client.internal.OriginSettingClient;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.ClusterStateTaskListener;
import org.elasticsearch.cluster.SimpleBatchedExecutor;
import org.elasticsearch.cluster.block.ClusterBlockException;
import org.elasticsearch.cluster.block.ClusterBlockLevel;
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.metadata.MetadataCreateIndexService;
import org.elasticsearch.cluster.routing.allocation.allocator.AllocationActionListener;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.cluster.service.MasterServiceTaskQueue;
import org.elasticsearch.common.Priority;
import org.elasticsearch.common.bytes.BytesReference;
import org.elasticsearch.common.compress.CompressedXContent;
import org.elasticsearch.common.inject.Inject;
import org.elasticsearch.common.settings.IndexScopedSettings;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.ThreadContext;
import org.elasticsearch.common.xcontent.XContentHelper;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.index.Index;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexNotFoundException;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.index.mapper.MappedFieldType;