## Problem
Exporting `file_cache_used` which specifies the number of used chunks in
the LFC. This helps calculate LFC utilization as: `file_cache_used_pages
/ (file_cache_used * file_cache_chunk_size_pages)`
## Summary of changes
Exporting `file_cache_used`.
Related Issue: https://github.com/neondatabase/cloud/issues/26688
