feat: webhook template

[baerwang]

No description provided.

[grobinson-grafana]

I appreciate this a popular request, but I am reluctant to support templating of JSON in Alertmanager. The main reason for this is that usability is very poor, and writing correct JSON in templating languages is very difficult once you start using conditionals and loops (i.e. trailing commas). I raised the same concerns on #3799 (comment). I'll leave it up to the other maintainers to make a decision on whether this is something we want in Alertmanager.

[Bezzzub]

We really need that feature. Give approve pls :)

[dmitryBLN]

The ability to independently generate a payload for webhooks is a necessary feature, and without it, using webhooks in alertmanager looks excessively curtailed compared to other integrations.

[TheMeier]

related #4243

[Amejonah1200]

The reason to wanting that feature would be to forward alerts to another alertmanager.

Instead of using go templating, why no use something like gojq instead?

[kunlun122]

We also need this feature too.

[TheMeier]

Even if templating the webhook payload would be accepted, I think it should be done in the existing webhook notifier and not in a new one

[SoloJacobs]

Hi,

thanks for this contribution. As already pointed out, we should update the existing webhook receiver. We are trying to implement this here: #5011

In case you have any use cases and specific requirements, feel free to comment on the PR.

Kind regards