Add support for postgres_exporter 13.0+

[szelenka]

Describe the enhancement you are proposing:
Consume the latest version from upstream for newest features and security patches. The upstream postgres_exporter introduced breaking changes, specifically to how queries.yaml is handled:
https://github.com/prometheus-community/postgres_exporter/releases/tag/v0.13.0

Specifically these PRs:
prometheus-community/postgres_exporter#801
prometheus-community/postgres_exporter#811

This breaks the way CrunchyData/pgmonitor handles the dynamic generation of queries to export data to Prometheus in the CrunchData/pgmonitor/postgres_exporter folder.

The above PRs appear to be taking a strong-arm approach to force contributions to the upstream prometheus-community/postgres_exporter repo, rather than maintaining individual queries.yaml files.

To keep using the prometheus-community/postgres_exporter upstream, it seems this project will need to contribute back the changes to upstream.

[keithf4]

Yes we are tracking this and will find a solution for this project. For now, you can continue using a version of the postgres_exporter that supports the custom query file feature.

[keithf4]

We will be migrating to using the sql_exporter in the future to solve this issue - #395