Review the need for absence alerts in prometheus alert example files

[keithf4]

Describe the enhancement you are proposing:

Most of the absence alerts can likely be removed due to the ExporterDown alert. If an exporter disappears, the ExporterDown alert would catch these.

One that may still be needed is the backrest absence alert per job/stanza since if a backup is completely missing in a repo that hosts multiple backups, you wouldn't know unless each one was defined.

See about moving the ExporterDown alert out of the node alerts file to its own "general" alerts file. Perhaps review some other built-in prometheus metrics to see if they can be included in this general alerts file.