Metrics

Add new annotations metrics #66:
- pgbackrest_backup_annotations
- pgbackrest_backup_last_annotations
Add new stanza metrics and bump default pgBackRest version to v2.48 #69:
- pgbackrest_stanza_lock_status
- pgbackrest_stanza_backup_compete_bytes
- pgbackrest_stanza_backup_total_bytes
Add exporter get data status metric #72:
- pgbackrest_exporter_status
Avoid flapping metrics #70, #74.
To avoid flapping time series, default values were set for several metrics. This includes setting a default value of 0 for metrics such as pgbackrest_backup_error_status, pgbackrest_backup_annotations, pgbackrest_backup_databases, etc. A block_incr label was also added to all *_backup_* and *_backup_last_* metrics.

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