Don't use a redundant file-filter for compressible-files

The messages function already only returns files with the right file
extension (eg. the messages), so we need only filter by the one property
we care about: compressibility.
Fix docs for archive/add-entry function
Only create DLO metrics once per subdirectory

This was previously creating the metrics every time a message was
discarded or the DLO was stored.
Add tests for DLO metrics