### Elasticsearch Version

8.1.2

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

ESS on Azure

### Problem Description

The `_xpack/usage` API reports the size of the data set per data tier. However, it uses the local disk size as the metric it reports on and this is 0 (zero) for partially mounted indices, hiding the size of the data set mounted on the frozen tier.

We should switch to using the `total_data_set_size` metric that was introduced when we added the frozen tier/partially mounted indices.

### Steps to Reproduce

Mount a partially mounted index on a dedicated frozen tier. Use the `_xpack/usage` API and see that no (or very little) data is reported on frozen tier.

### Logs (if relevant)

_No response_
x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/DataTiersUsageTransportActionTests.java
public void testCalculateMAD() {
        assertThat(DataTiersUsageTransportAction.computeMedianAbsoluteDeviation(new TDigestState(10)), equalTo(0L));
        TDigestState sketch = new TDigestState(randomDoubleBetween(1, 1000, false));
        sketch.add(1);
        sketch.add(1);
        sketch.add(2);
        sketch.add(2);
        sketch.add(4);
        sketch.add(6);
        sketch.add(9);
        assertThat(DataTiersUsageTransportAction.computeMedianAbsoluteDeviation(sketch), equalTo(1L));
    }
    public void testTierIndices() {
        IndexMetadata hotIndex1 = indexMetadata("hot-1", 1, 0, DataTier.DATA_HOT);