### 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
}, listener::onFailure));
    }
    
    
    static Map<String, String> tierIndices(ImmutableOpenMap<String, IndexMetadata> indices) {
        Map<String, String> indexByTier = new HashMap<>();
        indices.entrySet().forEach(entry -> {
            String tierPref = entry.getValue().getSettings().get(DataTier.TIER_PREFERENCE);
            if (Strings.hasText(tierPref)) {
                String[] tiers = tierPref.split(",");
                if (tiers.length > 0) {
                    indexByTier.put(entry.getKey(), tiers[0]);
                }
            }
        });
        return indexByTier;
    }
    /**
     * Accumulator to hold intermediate data tier stats before final calculation.
     */
    private static class TierStatsAccumulator {
        int nodeCount = 0;
        Set<String> indexNames = new HashSet<>();
        int totalShardCount = 0;
        long totalByteCount = 0;
        long docCount = 0;
        int primaryShardCount = 0;
        long primaryByteCount = 0L;
        final TDigestState valueSketch = new TDigestState(1000);
    }