Alert on logrotate errors
When gitlab logroate is unable to rotate (in one case because we previously had a full disk that left zero length files) we can get into a state where logs are never rotated which will eventually fill up the disk. It would be helpful in these situations to emit a metric for logrotate errors.
Example of errors in /var/log/gitlab/logrotate/current:
2018-06-03_11:30:37.51471 git-04 logrotate: error: error creating output file /var/log/gitlab/gitlab-rails/api_json.log.1: File exists
2018-06-03_11:30:37.51478 git-04 logrotate: error: error creating output file /var/log/gitlab/gitlab-rails/application.log.1: File exists
2018-06-03_11:30:37.51483 git-04 logrotate: error: error creating output file /var/log/gitlab/gitlab-rails/geo.log.1: File exists
2018-06-03_11:30:37.51491 git-04 logrotate: error: error creating output file /var/log/gitlab/gitlab-rails/grpc.log.1: File exists
2018-06-03_11:30:37.51496 git-04 logrotate: error: error creating output file /var/log/gitlab/gitlab-rails/production.log.1: File exists