Support go templating in alerting

[jetersen]

Describe the feature request

Would be nice instead of place holders if we could support full go templating in the alert providers if not all but at least the custom provider.

So we can do conditional logic and provide full templating.

https://pkg.go.dev/text/template

Why do you personally want this feature to be implemented?

All I want from our slack alert is to be able to include the URL for instance if status code is 500. Or I could template out the response body into the alert message.

How long have you been using this project?

2 weeks

Additional information

Would be willing to implement this myself.

[TwiN]

Any chance you could show me an example of how it would look in the configuration?

[jetersen]

Here is an example of how alert manager configures their slack notification via templates:
https://hodovi.cc/blog/creating-awesome-alertmanager-templates-for-slack/