### 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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.downsample;
import org.elasticsearch.ElasticsearchException;
import org.elasticsearch.ResourceAlreadyExistsException;
import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.ActionRequestValidationException;
import org.elasticsearch.action.DocWriteRequest;
import org.elasticsearch.action.admin.cluster.stats.MappingVisitor;
import org.elasticsearch.action.admin.indices.get.GetIndexResponse;
import org.elasticsearch.action.admin.indices.mapping.get.GetMappingsRequest;
import org.elasticsearch.action.admin.indices.mapping.get.GetMappingsResponse;
import org.elasticsearch.action.admin.indices.rollover.RolloverRequest;
import org.elasticsearch.action.admin.indices.rollover.RolloverResponse;
import org.elasticsearch.action.admin.indices.settings.put.UpdateSettingsRequest;
import org.elasticsearch.action.admin.indices.template.put.PutComposableIndexTemplateAction;
import org.elasticsearch.action.bulk.BulkItemResponse;
import org.elasticsearch.action.bulk.BulkRequestBuilder;
import org.elasticsearch.action.bulk.BulkResponse;
import org.elasticsearch.action.datastreams.CreateDataStreamAction;
import org.elasticsearch.action.datastreams.GetDataStreamAction;
import org.elasticsearch.action.index.IndexRequest;
import org.elasticsearch.action.support.WriteRequest;
import org.elasticsearch.action.support.master.AcknowledgedResponse;
import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
import org.elasticsearch.cluster.metadata.IndexMetadata;