databases/prometheus-postgres-exporter: Update to 0.19.1

Upstream changes:
https://github.com/prometheus-community/postgres_exporter/releases/tag/v0.19.0
https://github.com/prometheus-community/postgres_exporter/releases/tag/v0.19.1

Reviewed by: arrowd
Approved by: arrowd (ports)
Differential Revision: https://reviews.freebsd.org/D55701
Sponsored by: https://www.patreon.com/bsdivy