NOTE: pg_stat_bgwriter counter metrics had the `_total` suffix added #556

* [CHANGE] refactor pg_stat_bgwriter metrics into standalone collector #556
* [FEATURE] Add pg_database collector #613
* [ENHANCEMENT] Add pg_database_size_bytes metric #613
* [BUGFIX] Avoid parsing error from bogus Azure Flexible Server custom GUC #587
* [BUGFIX] Fix pg_stat_archiver error in 9.4 and earlier. #599
* [BUGFIX] Sanitize setting values because of Aurora irregularity #620

Signed-off-by: SuperQ <superq@gmail.com>