### 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
assertThat(warmStats.totalShardCount, is(2));
        assertThat(warmStats.docCount, is(2 * docCount));
        assertThat(warmStats.totalByteCount, is(2 * byteSize));
        assertThat(warmStats.primaryShardCount, is(1));
        assertThat(warmStats.primaryByteCount, is(byteSize));
        assertThat(warmStats.primaryByteCountMedian, is(byteSize)); 
        assertThat(warmStats.primaryShardBytesMAD, is(0L)); 
    }
    private static DiscoveryNode newNode(int nodeId, DiscoveryNodeRole... roles) {
        return new DiscoveryNode(
            "node_" + nodeId,
            ESTestCase.buildNewFakeTransportAddress(),
            Collections.emptyMap(),
            Set.of(roles),
            Version.CURRENT
        );
    }
    private static IndexMetadata indexMetadata(String indexName, int numberOfShards, int numberOfReplicas, String... dataTierPrefs) {
        Settings.Builder settingsBuilder = Settings.builder()
            .put(SETTING_VERSION_CREATED, Version.CURRENT)
            .put(SETTING_NUMBER_OF_SHARDS, numberOfShards)
            .put(SETTING_NUMBER_OF_REPLICAS, numberOfReplicas)
            .put(SETTING_CREATION_DATE, System.currentTimeMillis());
        if (dataTierPrefs.length > 1) {
            StringBuilder tierBuilder = new StringBuilder(dataTierPrefs[0]);
            for (int idx = 1; idx < dataTierPrefs.length; idx++) {
                tierBuilder.append(',').append(dataTierPrefs[idx]);
            }
            settingsBuilder.put(DataTier.TIER_PREFERENCE, tierBuilder.toString());
        } else if (dataTierPrefs.length == 1) {