### 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
long primaryShardSizeMedian = (long) accumulator.valueSketch.quantile(0.5);
        long primaryShardSizeMAD = computeMedianAbsoluteDeviation(accumulator.valueSketch);
        return new DataTiersFeatureSetUsage.TierSpecificStats(
            accumulator.nodeCount,
            accumulator.indexNames.size(),
            accumulator.totalShardCount,
            accumulator.primaryShardCount,
            accumulator.docCount,
            accumulator.totalByteCount,
            accumulator.primaryByteCount,
            primaryShardSizeMedian,
            primaryShardSizeMAD
        );
    }
    
    static long computeMedianAbsoluteDeviation(TDigestState valuesSketch) {
        if (valuesSketch.size() == 0) {
            return 0;
        } else {
            final double approximateMedian = valuesSketch.quantile(0.5);
            final TDigestState approximatedDeviationsSketch = new TDigestState(valuesSketch.compression());
            valuesSketch.centroids().forEach(centroid -> {
                final double deviation = Math.abs(approximateMedian - centroid.mean());
                approximatedDeviationsSketch.add(deviation, centroid.count());
            });
            return (long) approximatedDeviationsSketch.quantile(0.5);
        }
    }
}