### 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
if (randomBoolean()) {
                builder.field(FIELD_NUMERIC_2, randomDouble());
            }
            return builder.endObject();
        };
        bulkIndex(sourceSupplier);
        prepareSourceIndex(sourceIndex);
        rollup(sourceIndex, rollupIndex, config);
        assertRollupIndex(sourceIndex, rollupIndex, config);
    }
    public void testCannotRollupToExistingIndex() throws Exception {
        DownsampleConfig config = new DownsampleConfig(randomInterval());
        prepareSourceIndex(sourceIndex);
        
        assertAcked(
            client().admin()
                .indices()
                .prepareCreate(rollupIndex)
                .setSettings(Settings.builder().put("index.number_of_shards", 1).put("index.number_of_replicas", 0).build())
                .get()
        );
        ResourceAlreadyExistsException exception = expectThrows(
            ResourceAlreadyExistsException.class,
            () -> rollup(sourceIndex, rollupIndex, config)
        );
        assertThat(exception.getMessage(), containsString(rollupIndex));
    }
    public void testRollupEmptyIndex() throws IOException {
        DownsampleConfig config = new DownsampleConfig(randomInterval());