Feat: docs: development: testing: Prometheys unit testing (#102)

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## Testing

### Writing and trying unit tests for Prometheus

The [Prometheus][] alerts shipped by Onionprobe are tested by the `prom` [CI job][].

The configuration is available under the [configs/prometheus][] folder.

#### Workflow

A quick workflow to try tests before pushing to CI can be set with the

[standalone][] node:

    ./onionprobe-monitor up

    ./onionprobe-monitor shell prometheus

    promtool test rules /etc/prometheus/prometheus-tests.yml

[Prometheus]: https://prometheus.io/

[CI job]: https://docs.gitlab.com/ee/ci

[configs/prometheus]: https://gitlab.torproject.org/tpo/onion-services/onionprobe/-/tree/main/configs/prometheus

[standalone]: standalone.md

#### References

References for understanding and writing unit tests for [Prometheus][]:

* [Unit Testing for Rules | Prometheus](https://prometheus.io/docs/prometheus/latest/configuration/unit_testing_rules/)

* [Sleep Soundly: Reliable Alerting with Unit Testing in Prometheus | by Rubén Cougil Grande | Medium](https://medium.com/@rcougil/sleep-soundly-realiable-alerting-with-unit-testing-in-prometheus-260c652a3f9)

* [A Guide to Unit Testing Prometheus Alerts - Aviator Blog](https://www.aviator.co/blog/a-guide-to-unit-testing-prometheus-alerts/#)

* [Testing alerts - prometheus · Wiki · The Tor Project / TPA / TPA team · GitLab](https://gitlab.torproject.org/tpo/tpa/team/-/wikis/service/prometheus#testing-alerts)