Scrape CPU latency stats from /proc/schedstat (#1389)

These are useful as a direct indication of CPU contention and task
scheduler latency.

Handy references:
 - https://github.com/torvalds/linux/blob/master/Documentation/scheduler/sched-stats.txt
 - https://doc.opensuse.org/documentation/leap/tuning/html/book.sle.tuning/cha.tuning.taskscheduler.html

procfs is updated to pull in the enabling change:
https://github.com/prometheus/procfs/pull/186

Signed-off-by: Phil Frost <phil@postmates.com>

end-to-end-test.sh

@@ -30,6 +30,7 @@ enabled_collectors=$(cat << COLLECTORS
   nfsd
   pressure
   qdisc
+  schedstat
   sockstat
   stat
   textfile