feat(vmagent): add ansible role with docker support

roles/vmagent/README.md

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+# `finallycoffee.observability.vmagent` ansible role
+
+Install and configure the
+[victoriametrics agent `vmagent`](https://docs.victoriametrics.com/vmagent/)
+using the [supported deployment types (see `vars/main.yml#L5`)](vars/main.yml#L5).
+
+## Configuration
+
+Set scrape job configuration as complex data in `vmagent_config_scrape_configs`.
+To tune the scrape interval, override `vmagent_config_global_scrape_interval`,
+or modify / extend `vmagent_config` directly.
+
+### Prometheus remote write api with basic auth
+
+One of the more common methods of sending the collected metrics to a
+central prometheus server. Set the following variables to archieve this:
+
+```yaml
+vmagent_flags:
+  remoteWrite_url: https://my.prometheus.instance.example.com/api/v1/write
+  remoteWrite_basicAuth_username: my_prom_user
+  remoteWrite_basicAuth_passwordFile: /path/to/password/file.key
+```
+
+For the full set of options, see either the
+[vmagents' "Advanced usage" documentation](https://docs.victoriametrics.com/vmagent/#advanced-usage)
+or run `vmagent -help` for the same output.