use pushgateway to indicate pg backup start and finish
currently we capture wal-e basebackup message
Upload https://gitlab.com/gitlab-cookbooks/gitlab-mtail/blob/master/files/default/mtail/wale.mtail#L23 and use it in pgbasebackup alert to indicate if a base backup happened. However, this alert would never notify us if the backup failed during the process. Because of it, we missed 6 base backups in the last 14 days but never get any alert #7049 (closed).
We should use pushgateway before and after the backup to indicate the backup start and completion instead.