Update pgbouncer-exporter to v0.5.1
Update pgbouncer-exporter to v0.5.1

This contains minor fixes and testing for Go 1.18.

Modules are no longer vendored, so drop the `-mod=vendor` argument.

Relates to https://gitlab.com/groups/gitlab-org/-/epics/8843

Changelog: changed

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab