Improve Opsgenie integration

The current Opsgenie service is limited to sending message, and description. It would be great to be able to send additional optional fields especially priority, and tags, as well as dynamically setting the responders at the point the alert is generated.

I would be willing to contribute a PR for this functionality if there is interest in it.

Start by locating the current Opsgenie service and the alert-generation path that supplies its message and description fields. Trace how optional alert data is passed, then verify that priority, tags, and responders can be set when an alert is generated.

Written by the indexing model from the issue text.