### 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(FIELD_DIMENSION_2, randomIntBetween(1, 10))
                .field(FIELD_NUMERIC_1, randomInt())
                .field(FIELD_NUMERIC_2, DATE_FORMATTER.parseMillis(ts))
                .startObject(FIELD_AGG_METRIC)
                .field("min", randomDoubleBetween(-2000, -1001, true))
                .field("max", randomDoubleBetween(-1000, 1000, true))
                .field("sum", randomIntBetween(100, 10000))
                .field("value_count", randomIntBetween(100, 1000))
                .endObject()
                .field(FIELD_LABEL_DOUBLE, labelDoubleValue)
                .field(FIELD_METRIC_LABEL_DOUBLE, labelDoubleValue)
                .field(FIELD_LABEL_INTEGER, labelIntegerValue)
                .field(FIELD_LABEL_KEYWORD, ts)
                .field(FIELD_LABEL_UNMAPPED, randomBoolean() ? labelLongValue : labelDoubleValue)
                .field(FIELD_LABEL_TEXT, ts)
                .field(FIELD_LABEL_BOOLEAN, randomBoolean())
                .field(FIELD_LABEL_IPv4_ADDRESS, labelIpv4Address)
                .field(FIELD_LABEL_IPv6_ADDRESS, labelIpv6Address)
                .field(FIELD_LABEL_DATE, labelDateValue)
                .field(FIELD_LABEL_KEYWORD_ARRAY, keywordArray)
                .field(FIELD_LABEL_DOUBLE_ARRAY, doubleArray)
                .startObject(FIELD_LABEL_AGG_METRIC)
                .field("min", randomDoubleBetween(-2000, -1001, true))
                .field("max", randomDoubleBetween(-1000, 1000, true))
                .field("sum", Double.valueOf(randomIntBetween(100, 10000)))
                .field("value_count", randomIntBetween(100, 1000))
                .endObject()
                .endObject();
        };
        bulkIndex(sourceSupplier);