### 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/main/java/org/elasticsearch/xpack/core/DataTiersUsageTransportAction.java
static Map<String, DataTiersFeatureSetUsage.TierSpecificStats> calculateStats(
        List<NodeStats> nodesStats,
        Map<String, String> indexByTier,
        RoutingNodes routingNodes
    ) {
        Map<String, TierStatsAccumulator> statsAccumulators = new HashMap<>();
        for (NodeStats nodeStats : nodesStats) {
            aggregateDataTierNodeCounts(nodeStats, statsAccumulators);
            aggregateDataTierIndexStats(nodeStats, routingNodes, indexByTier, statsAccumulators);
        }
        Map<String, DataTiersFeatureSetUsage.TierSpecificStats> results = new HashMap<>();
        for (Map.Entry<String, TierStatsAccumulator> entry : statsAccumulators.entrySet()) {
            results.put(entry.getKey(), calculateFinalTierStats(entry.getValue()));
        }
        return results;
    }
    /**
     * Determine which data tiers this node belongs to (if any), and increment the node counts for those tiers.
     */
    private static void aggregateDataTierNodeCounts(NodeStats nodeStats, Map<String, TierStatsAccumulator> tiersStats) {
        nodeStats.getNode()
            .getRoles()
            .stream()
            .map(DiscoveryNodeRole::roleName)
            .filter(DataTier::validTierName)
            .forEach(tier -> tiersStats.computeIfAbsent(tier, k -> new TierStatsAccumulator()).nodeCount++);
    }
    /**
     * Locate which indices are hosted on the node specified by the NodeStats, then group and aggregate the available index stats by tier.