[BUG] Metric backmon_backup_file_young_count reports incorrectly

Given the following definitions file
```yaml
'./pfsense-backup':
defaults:
schedule: 05 04 * * *
purge: false
files:
'%Y%M%D-config.xml':
alias: pfSense
retention-count: 14
retention-age: 14d
'./freeipa-backup/{{date}}':
fuse:
- 'date'
defaults:
schedule: 30 21 * * *
purge: false
files:
'header':
retention-count: 14
retention-age: 14d
'ipa-full.tar':
retention-count: 14
retention-age: 14d
```

Backmon reports the following metrics:
```
# HELP backmon_backup_file_young_count The amount of backup files in this group that are younger than the maximum age (file_age_aim_seconds).
# TYPE backmon_backup_file_young_count gauge
backmon_backup_file_young_count{dir="./freeipa-backup/{{date}}",disk="ws3-backup",file="header",group="freeipa-backup/{{date}}"} 0
backmon_backup_file_young_count{dir="./freeipa-backup/{{date}}",disk="ws3-backup",file="ipa-full.tar",group="freeipa-backup/{{date}}"} 0
backmon_backup_file_young_count{dir="./pfsense-backup",disk="ws3-backup",file="pfSense",group="pfsense-backup"} 0
```

However, since _retention-age_ is set, the metric should report properly (value `0` for each metric is probably wrong).

Reproduce the reported metrics with the YAML definitions in the issue, then trace the backmon code that computes backmon_backup_file_young_count and applies retention-age. Done means the metric reports the correct young-file count for each listed backup file, with coverage for the provided configuration.

Written by the indexing model from the issue text.