Update pgbouncer metric names
In preparation for pgbouncer_exporter v0.4.0.
Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13555
The result dashboard: https://dashboards.gitlab.net/dashboard/snapshot/1T54rG8Nv1RPJlTvMJDM5BVf0bDD5tsq
Edited by Ahmad Sherif