### 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
shard.shardId(),
            rollupIndex,
            config,
            new String[] { FIELD_NUMERIC_1, FIELD_NUMERIC_2 },
            new String[] {}
        );
        /*
         * Here we set the batch size and the total bytes in flight size to tiny numbers so that we are guaranteed to trigger the bulk
         * processor to reject some calls to add(), so that we can make sure RollupShardIndexer keeps trying until success.
         */
        indexer.rollupMaxBytesInFlight = ByteSizeValue.ofBytes(1024);
        indexer.rollupBulkSize = ByteSizeValue.ofBytes(512);
        indexer.execute();
    }
    private DateHistogramInterval randomInterval() {
        return ConfigTestHelpers.randomInterval();
    }
    private String randomDateForInterval(DateHistogramInterval interval) {
        long endTime = startTime + MAX_NUM_BUCKETS * interval.estimateMillis();
        return randomDateForRange(startTime, endTime);
    }
    private String randomDateForRange(long start, long end) {
        return DATE_FORMATTER.formatMillis(randomLongBetween(start, end));
    }
    private void bulkIndex(SourceSupplier sourceSupplier) throws IOException {
        bulkIndex(sourceIndex, sourceSupplier);
    }
    private void bulkIndex(String indexName, SourceSupplier sourceSupplier) throws IOException {
        BulkRequestBuilder bulkRequestBuilder = client().prepareBulk();
        bulkRequestBuilder.setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE);