feat(cadvisor): add ansible role for deployment using docker containers

roles/cadvisor/README.md

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+# `finallycoffee.observability.cadvisor` ansible role
+
+## Overview
+
+Deploys [cadvisor](https://github.com/google/cadvisor/), a daemon
+for collecting and exporting information about running (docker)
+containers in a docker container.
+
+## Configuration
+
+In order to scrape `/metrics` of running containers, it is recommended
+to expose the default port of cadvisor to the host using
+```yaml
+cadvisor_container_ports:
+  - "127.0.0.1:8080:8080`
+```
+so that cadvisor metrics are exposed at `http://127.0.0.1:8080/metrics`.
+
+### Enabling/Disabling collection of metrics
+
+By setting `cadvisor_disabled_metrics`, the collection of metrics
+can be disabled. The default list of disabled metrics is quite extensive,
+so when enabling a disabled-by-default metric, it is recommended to
+use `cadvisor_force_enable_metrics` instead, as it's empty by default.