Feature request: Btrfs commit stats

Since Linux 6.0, we can get commit statistics from `/sys/fs/btrfs/FS_ID/commit_stats`, e.g.

```console
$ cat /sys/fs/btrfs/01234567-89ab-cdef-0123-456789abcdef/commit_stats
commits 258051
last_commit_ms 1000
max_commit_ms 51462
total_commit_ms 47836090
```