### 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
public static final String FIELD_LABEL_KEYWORD = "label_keyword";
    public static final String FIELD_LABEL_TEXT = "label_text";
    public static final String FIELD_LABEL_BOOLEAN = "label_boolean";
    public static final String FIELD_LABEL_IPv4_ADDRESS = "label_ipv4_address";
    public static final String FIELD_LABEL_IPv6_ADDRESS = "label_ipv6_address";
    public static final String FIELD_LABEL_DATE = "label_date";
    public static final String FIELD_LABEL_UNMAPPED = "label_unmapped";
    public static final String FIELD_LABEL_KEYWORD_ARRAY = "label_keyword_array";
    public static final String FIELD_LABEL_DOUBLE_ARRAY = "label_double_array";
    public static final String FIELD_LABEL_AGG_METRIC = "label_agg_metric";
    private static final int MAX_DIM_VALUES = 5;
    private static final long MAX_NUM_BUCKETS = 10;
    private String sourceIndex, rollupIndex;
    private long startTime;
    private int docCount, numOfShards, numOfReplicas;
    private List<String> dimensionValues;
    @Override
    protected Collection<Class<? extends Plugin>> getPlugins() {
        return List.of(
            LocalStateCompositeXPackPlugin.class,
            Rollup.class,
            AggregateMetricMapperPlugin.class,
            DataStreamsPlugin.class,
            IndexLifecycle.class
        );
    }
    @Before
    public void setup() throws IOException {
        sourceIndex = randomAlphaOfLength(14).toLowerCase(Locale.ROOT);
        rollupIndex = "rollup-" + sourceIndex;