Add MS Teams webhook support

[darkowlzz]

Refer: Actionable Messages via Connectors

The default theme color used for firing is #F35225 (Red in MS Logo), and resolved is #81BB05 (Green in MS Logo), to match with their theme, maybe 😄

Sample payload sent to Teams:

{
  "@context": "http://schema.org/extensions",
  "@type": "MessageCard",
  "title": "Alert from System 7",
  "text": "[FIRING:1]  (TURTLE ALERT!!)",
  "themeColor": "F35225",
  "potentialAction": [
    {
      "@type": "OpenUri",
      "name": "View Alert",
      "targets": [
        {
          "os": "default",
          "uri": "http://localhost:9093/#/alerts?receiver=some-teams-rcvr"
        }
      ]
    }
  ]
}

Sample Alertmanager config for the same:

global:
  teams_api_url: "https://outlook.office.com/webhook/somekey/IncomingWebhook/someotherkey"

receivers:
  - name: "some-teams-rcvr"
    teams_configs:
      - send_resolved: true
        # api_url: ""
        title: "Alert from System 7"

[darkowlzz]

CONTRIBUTING.md:

If you have a trivial fix or improvement, go ahead and create a pull request, addressing (with @...) the maintainer of this repository (see MAINTAINERS.md) in the description of the pull request.

@stuartnelson3 😊

[sahilsk]

Let's get this merge. I was waiting for this integration. 🙏

[stuartnelson3]

thanks for the contribution!

unfortunately, as you can see in prometheus/docs#843, we're currently not adding any new notifiers. Our recommendation is to implement additional integrations with the webhook notifier.

[sengaya]

I wonder if this PR could be reopened, now that recently Telegram support was added and the moratorium was lifted.

[roidelapluie]

Probably not going to reopen, but feel free to work off the original commit and rebase it.