added mtail script to keep track of WAL-E backups to S3
As part of https://gitlab.com/gitlab-com/infrastructure/issues/1095, this mtail script would be able to track the number of successful WAL-E archivals in a counter.
As part of https://gitlab.com/gitlab-com/infrastructure/issues/1095, this mtail script would be able to track the number of successful WAL-E archivals in a counter.