Support webhook_url_file for discord config

[josedev-union]

What did you do?
I use a main Discord for receiver.
What did you expect to see?
I want to use pre-existing K8s secret to configure Discord receiver's webhook url instead of setting that as helm value. Like slack_api_url_file, mount the existing k8s secret into alertmanager and use that file as webhook_url.

[Roronoa-Zoroo]

the documentation mentions the use of webhook_url_file for discord_config
i was pulling my hair as to why it was not working

[arichtman]

The configuration documentation does mention this option, but when I try to provide just webhook_url_file it complains that no webhook_url is provided, and if I provide both, it exclaims that only one is permissible.

[tuana9a]

@arichtman same here, just found out that this has been fixed in #3728 but it hasn't been released yet.

[arichtman]

v0.28.0 seems to have the fix.