Use push_metrics instead of milemarker

[aelkiss]

https://github.com/hathitrust/push_metrics

This way we should get alerts when the job fails for "free".

If we want to measure any other metrics we could do that too but I think we shuold start with getting basic job completion info.

[aelkiss]

This is already being used, but we should investigate why we aren't getting alerts when hathifiles aren't generated.

[aelkiss]

Looks like it may be reporting success even though it somehow failed? Not sure... Possible we're calling log_final_line (and thus indicating job success) even if there's an exception that prevents completion?