Function collect_unit_files_stats 

pub async fn collect_unit_files_stats(fs_root: &str) -> UnitFilesStats

Collect unit file statistics from the filesystem. fs_root is prepended to all paths — empty string for the host, /proc/<pid>/root for containers.

All directory reads are issued in parallel: the three generator directories, the root transient directory, and user-dir enumeration run concurrently in a first batch; per-user transient reads run concurrently in a second batch.