feat: Add btrfs discard stats

I see my drives are issuing lots of discards while moving data (unsurprisingly). It'd be nice if the node_exporter could pick up these stats

Looks like this would probably need added to https://github.com/prometheus/procfs and would be similar to commit_stats https://github.com/prometheus/procfs/pull/580

root@machine:/sys/fs/btrfs/0dee6821-7166-4f43-adc1-cf871ec53ef4# ls -al discard/
total 0
drwxr-xr-x 2 root root    0 Mar 22 17:08 .
drwxr-xr-x 7 root root    0 Mar 22 16:47 ..
-r--r--r-- 1 root root 4096 Mar 22 17:08 discard_bitmap_bytes
-r--r--r-- 1 root root 4096 Mar 22 17:08 discard_bytes_saved
-r--r--r-- 1 root root 4096 Mar 22 17:08 discard_extent_bytes
-r--r--r-- 1 root root 4096 Mar 22 17:08 discardable_bytes
-r--r--r-- 1 root root 4096 Mar 22 17:08 discardable_extents
-rw-r--r-- 1 root root 4096 Mar 22 17:08 iops_limit
-rw-r--r-- 1 root root 4096 Mar 22 17:08 kbps_limit
-rw-r--r-- 1 root root 4096 Mar 22 17:08 max_discard_size

Start by reviewing the node_exporter filesystem collector and the related prometheus/procfs commit_stats implementation, then inspect the btrfs discard directory under /sys/fs/btrfs//discard. Done means the listed discard statistics are read through procfs and exposed by node_exporter with appropriate coverage.