[prometheus-postgres-exporter] optionally include mixin content

[jcpunk]

Is your feature request related to a problem ?

Add a flag in values.yaml to include the mixin content. The upstream has example dashboards and alerts that would be handy to seed if requested.

Describe the solution you'd like.

Render the mixins and maybe setup renovate to track them?

Describe alternatives you've considered.

Hand maintenance

Additional context.

https://github.com/prometheus-community/postgres_exporter/tree/master/postgres_mixin