Data tiers usage is 0 for partially mounted indices #86055
Description
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