### 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
prepareSourceIndex(sourceIndex);
        Exception exception = expectThrows(ActionRequestValidationException.class, () -> rollup(sourceIndex, rollupIndex, config));
        assertThat(exception.getMessage(), containsString("does not contain any metric fields"));
    }
    public void testCannotRollupWriteableIndex() {
        DownsampleConfig config = new DownsampleConfig(randomInterval());
        
        Exception exception = expectThrows(ElasticsearchException.class, () -> rollup(sourceIndex, rollupIndex, config));
        assertThat(exception.getMessage(), containsString("Rollup requires setting [index.blocks.write = true] for index"));
    }
    public void testCannotRollupMissingIndex() {
        DownsampleConfig config = new DownsampleConfig(randomInterval());
        IndexNotFoundException exception = expectThrows(IndexNotFoundException.class, () -> rollup("missing-index", rollupIndex, config));
        assertEquals("missing-index", exception.getIndex().getName());
        assertThat(exception.getMessage(), containsString("no such index [missing-index]"));
    }
    public void testCannotRollupWhileOtherRollupInProgress() throws Exception {
        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);
        var rollupListener = new ActionListener<AcknowledgedResponse>() {
            boolean success;
            @Override
            public void onResponse(AcknowledgedResponse acknowledgedResponse) {