### 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
{
            IndexRoutingTable.Builder indexRoutingTableBuilder = IndexRoutingTable.builder(warmIndex1.getIndex());
            routeTestShardToNodes(warmIndex1, 0, indexRoutingTableBuilder, mixedNode1, mixedNode2);
            routingTableBuilder.add(indexRoutingTableBuilder.build());
        }
        IndexMetadata warmIndex2 = indexMetadata("warm_index_2", 1, 1, DataTier.DATA_WARM);
        metadataBuilder.put(warmIndex2, false).generateClusterUuidIfNeeded();
        {
            IndexRoutingTable.Builder indexRoutingTableBuilder = IndexRoutingTable.builder(warmIndex2.getIndex());
            routeTestShardToNodes(warmIndex2, 0, indexRoutingTableBuilder, mixedNode3, mixedNode1);
            routingTableBuilder.add(indexRoutingTableBuilder.build());
        }
        
        ClusterState clusterState = ClusterState.builder(new ClusterName("test"))
            .nodes(discoBuilder)
            .metadata(metadataBuilder)
            .routingTable(routingTableBuilder.build())
            .build();
        long byteSize = randomLongBetween(1024L, 1024L * 1024L * 1024L * 30L); 
        long docCount = randomLongBetween(100L, 100000000L); 
        List<NodeStats> nodeStatsList = buildNodeStats(clusterState, byteSize, docCount);
        
        Map<String, String> indexByTier = DataTiersUsageTransportAction.tierIndices(clusterState.metadata().indices());
        Map<String, DataTiersFeatureSetUsage.TierSpecificStats> tierSpecificStats = DataTiersUsageTransportAction.calculateStats(
            nodeStatsList,
            indexByTier,
            clusterState.getRoutingNodes()
        );