mattermost: flatten attachments into top-level config, keep BC

[madest92]

This PR refactors the Mattermost receiver configuration by flattening the attachment structure(like slack_configs).

Previously, message formatting required configuring fields inside attachments[0].
Now, commonly used fields such as title, title_link, color, and text are promoted to the top level of mattermost_configs.

Additionally, more default templates are provided to reduce the need for repetitive manual configuration.

Example with default template:

receivers:
- name: 'mm'
  mattermost_configs:
  - send_resolved: false

Breaking change

This change modifies the configuration schema of mattermost_configs.

Old configuration:

mattermost_configs:
  - text: ''
    attachments:
      - color: '{{ if eq .Status "firing" }}danger{{ else }}good{{ end }}'
        title: '{{ .CommonAnnotations.summary }}'
        title_link: '{{ template "__alertmanagerURL" . }}'
        text: '{{ range .Alerts }}{{ .Annotations.description }}{{ end }}'

New configuration:

mattermost_configs:
  - title: '{{ .CommonAnnotations.summary }}'
    title_link: '{{ template "__alertmanagerURL" . }}'
    color: '{{ if eq .Status "firing" }}danger{{ else }}good{{ end }}'
    text: '{{ range .Alerts }}{{ .Annotations.description }}{{ end }}'

Users must update existing configurations accordingly.

[madest92]

@TheMeier hi
We discussed this here

[TheMeier]

Why is asset/assets_vfsdata.go here?

[TheMeier]

Users must update existing configurations accordingly.

That does not sound good to me

[madest92]

Why is asset/assets_vfsdata.go here?

I updated the templates. Without this, alertmanager would not see new templates.

[madest92]

That does not sound good to me

I know. Me too. But the current configuration isn't convenient via attachments. I still use slack_configs.

global:
  slack_api_url: '<mattermost_webhook>'

receivers:
  - name: 'critical'
    slack_configs:
    - send_resolved: true
      title: '{{...}}'
      text: '{{...}}'

Mattermost was introduced not long ago, so I don't think it's a big problem.

[siavashs]

That does not sound good to me

I know. Me too. But the current configuration isn't convenient via attachments. I still use slack_configs.
...
Mattermost was introduced not long ago, so I don't think it's a big problem.

That is not a good excuse, we need to make changes backwards compatible.
The best approach is to deprecate the old fields and introduce the new ones.
Eventually we can remove the deprecated fields in a future release where we announce the breaking change.

[madest92]

That is not a good excuse, we need to make changes backwards compatible.

Agreed. I'll make it backwards compatible.

[madest92]

@siavashs

I added backward compatibility

1. Old behavior when user uses attachment

2. New behavior without using attachment. Default templates are already in use

[madest92]

@TheMeier @siavashs hi guys.

Can you please look at the merge again?

Anything else needed?