Add metrics for btrfs commit statistics

[maartenberg]

Add four metrics to the btrfs collector to expose statistics on the commits to the filesystem: node_btrfs_commits_total, node_btrfs_last_commit_seconds, node_btrfs_max_commit_seconds, and node_btrfs_commit_seconds_total.

These values were added to the procfs library in https://github.com/prometheus/procfs/pull/580, but not exposed as a metric yet.

Fixes https://github.com/prometheus/node_exporter/issues/2734.

[maartenberg]

Ping @SuperQ @discordianfish: could you still take a look at this / is there anything else I should do?