postgres_exporter: Update build and queries

Created by: efritz

I tried experimenting with postgres exporter and the Postgres dashboard a bit. Some of these ideas may stick and some may not - let me know if reviewers would like this to be broken down more.

• Upgraded postgres_exporter from 0.7.0 to 0.9.0 (it has since changed ownership to prometheus-community)
• Split the postgres_exporter queries configuration into multiple files so we can source them in a more manageable way
• Added dashboards to track table and index size and bloat