Monitoring of WAL/xlog files for PostgreSQL

Per https://gitlab.com/gitlab-com/infrastructure/issues/1448#note_27283592 we should add more monitoring for WAL files, archiving problems, etc:

Monitor WAL/xlog files which are ready to be archived (count of files in pg_xlog/archive_status which end in ".ready")

• Point-in-time recovery requires WAL files. If many WAL files are not yet archived and a failure occurs then the backup will be missing, potentially, a lot of WAL.

• WAL files not being archived may be indicative of a problem with the archive_command

• Unarchived WAL files may continue to accumulate and result in disk space issues