feat(postgres_exporter): add ansible role for deployment using docker

roles/postgres_exporter/README.md

@@ -0,0 +1,18 @@
+# `finallycoffee.observability.postgres_exporter` ansible role
+
+## Overview
+
+Ansible role to deploy [`postgres_exporter`](https://github.com/prometheus-community/postgres_exporter),
+running in a docker container.
+
+## Configuration
+
+Set `postgres_exporter_db_[host|post|user|pass]` according to your
+databases configuration, and scrape the container on it's port `9187`
+(e.g.: `http://{container_ip}:9187/metrics`).
+
+For more configuration options using environment variables, see the
+[official README](https://github.com/prometheus-community/postgres_exporter)
+and set the configuration in `postgres_exporter_container_env` to override
+the defaults.
+