Integration with Prometheus / Alertmanager / Grafana monitoring

**Is your feature request related to a problem? Please describe.**
No, it isn't.

**Describe the solution you'd like**
Nowadays it's really common to use Prometheus / Alertmanager / Grafana stack for monitoring and alerting. All components are written in Go and they are open-source.
The notification / alerts in case of events / errors can be configured in 2 places:

**Alertmanager** (the recommended place)
* It doesn't has integration with Gotify and the the authors are not interested. => https://github.com/prometheus/alertmanager/issues/2120
* They provide a Webhook mechanism to send notifications via POST request.
* Some users have implemented they own bridges (take a look at the previous issue). I'm using => https://github.com/DRuggeri/alertmanager_gotify_bridge

**Grafana**
* Same issues as Alertmanager. They don't have Gotify support but they provide a Webhook.
* It looks like some users have Grafana => Gotify notifications working but they are not configurable enough. => https://github.com/gotify/server/issues/312

**Feature request**
I think those software are really popular and the integration with Gotify should be easier to configure.
* Gotify should provide native integration for those Webhooks or an official plugin. This project is working but the author was not able to convert the server into a plugin => https://github.com/DRuggeri/alertmanager_gotify_bridge
* Improve documentation about Alertmanager / Grafana integration. At least add some notes.