### 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
for (int i = 0; i < docCount; i++) {
            IndexRequest indexRequest = new IndexRequest(indexName).opType(DocWriteRequest.OpType.CREATE);
            XContentBuilder source = sourceSupplier.get();
            indexRequest.source(source);
            bulkRequestBuilder.add(indexRequest);
        }
        BulkResponse bulkResponse = bulkRequestBuilder.get();
        int duplicates = 0;
        for (BulkItemResponse response : bulkResponse.getItems()) {
            if (response.isFailed()) {
                if (response.getFailure().getCause() instanceof VersionConflictEngineException) {
                    
                    
                    logger.debug("We tried to insert a duplicate: [{}]", response.getFailureMessage());
                    duplicates++;
                } else {
                    fail("Failed to index data: " + bulkResponse.buildFailureMessage());
                }
            }
        }
        int docsIndexed = docCount - duplicates;
        logger.info("Indexed [{}] documents. Dropped [{}] duplicates.", docsIndexed, duplicates);
        assertHitCount(client().prepareSearch(indexName).setSize(0).get(), docsIndexed);
    }
    private void prepareSourceIndex(String sourceIndex) {
        
        assertAcked(
            client().admin()
                .indices()
                .prepareUpdateSettings(sourceIndex)