New compression stats require restic repo v2 format and compression enabled

[DrMurx]

The global stats introduced in 01b0b14 require that the underlying restic repository have been created or migrated to the version 2 data format, AND have compression actually enabled.

1. For repos that are still version 1, the restic stats --mode raw-data response lacks the total_uncompressed_size key and the exporter fails with

Unable to collect metrics from Restic. KeyError: 'total_uncompressed_size'

Migrating to v2 is fairly easy (restic migrate upgrade_repo_v2), but the exporter's error could be more verbose.

2. Once migrated to v2, restic stats --mode raw-data still lacks a few new fields. These stats will only appear after a backup with --compression max has been performed at least once.