Add: alertmanager-mattermost in k8s document

Signed-off-by: Ali Mehraji <a.mehraji75@gmail.com>

Author: cod3mas0n

posts/alertmanager-mattermost-k8s-prometheus.md

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+---
+title: Mattermost Receiver For Alertmanager in Prometheus-Stack in K8S
+published: true
+description: Add Mattermost Receiver for alertmanager in prometheus-stack in k8s
+tags: 'alertmanager,prometheus,monitoring,kubernetes'
+---
+
+### Why
+
+Since Prometheus-Stack Alertmanager doesn’t include Mattermost as a built-in receiver, we will integrate it using a webhook via creating a Python relay application that accepts Alertmanager webhook notifications, formats them into a Mattermost-compatible payload, and sends them to a Mattermost incoming webhook URL.
+
+Then we will deploy this relay inside Kubernetes using a Deployment and expose it with a Service so Alertmanager can reliably reach it.
+
+Finally, we will configure Alertmanager using an AlertmanagerConfig resource to define a new receiver and route, so alerts matching our rules (severity/namespace/etc.) get delivered to Mattermost through the relay.
+
+### How
+
+- Prometheus Alert Rules trigger alerts.
+- Alertmanager processes and routes the alerts.
+- Alertmanager sends webhook notifications to the Python Relay Service.
+- The relay formats the alert payload.
+- The relay forwards the formatted message to Mattermost Incoming Webhook.
+- Alerts appear in the designated Mattermost channel.
+
+### Steps
+
+- Create an [Incoming Webhook in Mattermost][mattermost-incoming-webhook]. This webhook URL will be used by the Python relay to deliver alert messages to the desired channel.
+   > Ensure you securely store the generated webhook URL, as it will be injected into the relay application via environment variables or Kubernetes secrets.
+
+- [Python Relay][python-relay-app] Application
+
+    > A lightweight webhook server that receives Alertmanager alerts and forwards them to Mattermost.
+
+- Build the Docker Image [Dockerfile][dockerfile]
+
+    Once the relay application is complete, build the image and push it to a container registry accessible by your Kubernetes cluster.
+
+- Deploy in Kubernetes: [K8S Manifests][k8s-manifests]
+- Configure Alertmanager with [AlertmanagerConfig][alertmanagerconfig-manifest]
+   > Ensure having a [label][alertmanagerconfig-label] in AlertmanagerConfig to select via prometheus operator
+
+- Add the Alertmanagerconfig to prometheus `values.yaml`
+
+    ```yaml
+    ⋮
+    alertmanager:
+    alertmanagerSpec:
+        alertmanagerConfigSelector:
+        matchLabels:
+            alertmanagerConfig: alertmanager-mattermost
+    ⋮
+    ```
+
+[mattermost-incoming-webhook]:https://docs.mattermost.com/integrations-guide/incoming-webhooks.html
+[python-relay-app]: https://github.com/cod3mas0n/k8s-alertmanager-mattermost/blob/main/src/app.py
+[dockerfile]: https://github.com/cod3mas0n/k8s-alertmanager-mattermost/blob/main/src/Dockerfile
+[k8s-manifests]: https://github.com/cod3mas0n/k8s-alertmanager-mattermost/tree/main/kubernetes
+[alertmanagerconfig-manifest]: https://github.com/cod3mas0n/k8s-alertmanager-mattermost/blob/main/kubernetes/alertmanagerConfig.yaml
+[alertmanagerconfig-label]: https://github.com/cod3mas0n/k8s-alertmanager-mattermost/blob/main/kubernetes/alertmanagerConfig.yaml#L6-L7