### 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
field -> assertTrue(
                                    "Field [" + field + "] is not included in the source fields: " + originalFieldsList,
                                    originalFieldsList.contains(field)
                                )
                            );
                            Object originalLabelValue = originalHit.getDocumentFields().values().stream().toList().get(0).getValue();
                            Object rollupLabelValue = rollupHit.getDocumentFields().values().stream().toList().get(0).getValue();
                            Optional<Aggregation> labelAsMetric = nonTopHitsOriginalAggregations.stream()
                                .filter(agg -> agg.getName().equals("metric_" + rollupTopHits.getName()))
                                .findFirst();
                            
                            if (labelAsMetric.isPresent()) {
                                double metricValue = ((Max) labelAsMetric.get()).value();
                                assertEquals(metricValue, rollupLabelValue);
                                assertEquals(metricValue, originalLabelValue);
                            }
                        }
                    }
                }
            }
        });
    }
    @SuppressWarnings("unchecked")
    private void assertFieldMappings(
        DownsampleConfig config,
        Map<String, TimeSeriesParams.MetricType> metricFields,
        Map<String, Map<String, Object>> mappings
    ) {
        
        assertEquals(DateFieldMapper.CONTENT_TYPE, mappings.get(config.getTimestampField()).get("type"));