feat(postgres-system): Add monitoring dashboard for postgres-exporter

This patch adds a sashboard to monitor postgresql instances on the
cluster. Currently the instance need to specify the sidecar container
themselves along with a pod monitor. See matrix for reference.

References:
https://git.shivering-isles.com/shivering-isles/infrastructure-gitops/-/blob/3c6b1410b0e0edfa1d331f96e1a8836b33590d57/apps/base/matrix/database.yaml#L31-76