Adding cluster label docs

docs/setup-robusta/additional-settings.rst

@@ -1,5 +1,3 @@
-:hide-toc:
-
 Additional Settings
 =======================
 
@@ -221,3 +219,29 @@ Add this to Robusta's Helm values:
               memory: 2048Mi
             limits:
               memory: 2048Mi
+
+Adding a Cluster Label to Alerts
+---------------------------------------------
+When using Robusta's built-in Prometheus stack, Robusta uses the ``cluster_name`` you set during installation to identify which alerts belong to which cluster.
+
+If you forward external alerts to Robusta (e.g., from Grafana/Grafana Cloud), you will need to pass the ``cluster_name`` metadata manually. For example, if you use Grafana alerting, ensure that all your metrics and alerts have a ``cluster_name`` label.
+
+You can add this label to all of your metrics using ``prometheus.prometheusSpec.additionalScrapeConfigs`` as mentioned below. Add the following config to your Helm values file.
+
+.. code-block:: yaml
+
+    prometheus:
+      prometheusSpec:
+        additionalScrapeConfigs:
+          - job_name: "cluster-name-to-metric"
+            kubernetes_sd_configs:
+              - role: pod
+            metric_relabel_configs:
+              - target_label: cluster_name
+                replacement: "YOUR_ROBUSTA_CLUSTER_NAME" # This is the cluster name you set in the Helm values during Robusta installation
+
+.. note:: 
+
+  1. ``cluster_name`` label will be added only to metrics after you add this config. i.e Previously scraped metrics will not have ``cluster_name`` label. **You will need to wait a few hours after adding this configuration for the label to show up on your alerts and be forwarded correctly.**
+  2. ``prometheus.prometheusSpec.externalLabels.cluster`` does not work for cases when you need ``cluster_name`` label in Grafana.
+