As Praefect talks to PostgeSQL instance we should use PgBouncer
in front of database to reduce resource consumption.
All required changes already done https://gitlab.com/gitlab-org/gitaly/-/issues/2816
in Praefect and tested https://gitlab.com/gitlab-org/gitaly/-/issues/2975
on the weekly demo.
The change describes required options that should be set for the
PgBouncer instance and what values should be substituted so
Praefect instances could connect to PostgreSQL throw PgBouncer.

Closes: https://gitlab.com/gitlab-org/gitaly/-/issues/2976