Rework the postgres_mixing dashboard to be more composable. The goal is for this to be more maintainable long term. I don't know jsonnet very well, but following other projects, this appears to be in line.
This replaces the postgres-overview.json dashboard with an overview.json dashboard with the same panels. While the dashboard does not match perfectly, it does include the same data but with the correct metrics.
Signed-off-by: Joe Adams <github@joeadams.io>
