Add multiple gitlab-monitor DB metrics to postgres-exporter
This ports metrics from the following database collectors in gitlab-monitor:
- BlockedQueriesCollector
- SlowQueriesCollector
- StuckIdleInTransactionsCollector
- VacuumQueriesCollector
The ported queries sometimes have to look somewhat different to work in the postgres-exporter, and hopefully they have the same or close-enough outcomes as in gitlab-monitor.
Some of the metric names didn't adhere to best practices in gitlab-monitor (like "_total" as a suffix for gauges), but for now I'm keeping them the same in postgres-exporter to keep the number of changing things lower.