Add script for collecting borgmatic backup metrics

[g00g1]

This is a script for borgmatic, adds following metrics:

• borgmatic_backup_success: whether a borgmatic operation succeeded
• borgmatic_backup_duration_seconds: duration of a borgmatic operation
• borgmatic_backup_latest_archive_files: number of files within latest backup archive
• borgmatic_backup_latest_archive_original_size_bytes: original size of a latest backup archive
• borgmatic_backup_latest_archive_compressed_size_bytes: compressed size of a latest backup archive
• borgmatic_backup_latest_archive_deduplicated_size_bytes: deduplicated size of a latest backup archive

[dswarbrick]

Thanks for your contribution.

We are trying to discourage the use of homegrown text exposition format rendering. Please use the official Prometheus Python client library (https://github.com/prometheus/client_python) instead.