Update monitoring components
Update various monitoring components to current releases
- Prometheus 2.15.2
- Alertmanager 0.20.0
- pgbouncer-exporter 0.1.2-gitlab
- postgres-exporter 0.8.0
- redis-exporter 1.3.5
Update postgres-exporter query based on upstream example.
Signed-off-by: Ben Kochie bjk@gitlab.com