[ROB-2864] gcp docs update

docs/data-sources/builtin-toolsets/prometheus.md

@@ -197,6 +197,33 @@ holmes:
 
 ---
 
+## Google Managed Prometheus Configuration
+
+Before configuring Holmes, make sure you have:
+
+* Google Managed Prometheus enabled
+* A Prometheus Frontend endpoint accessible from your cluster
+  (If you don’t already have one, you can create it following the instructions
+  [here](https://docs.cloud.google.com/stackdriver/docs/managed-prometheus/query-api-ui#ui-prometheus) )
+
+To connect HolmesGPT to Google Cloud Managed Prometheus:
+
+```yaml
+holmes:
+  toolsets:
+    prometheus/metrics:
+      enabled: true
+      config:
+        # Set this to the URL of your Prometheus Frontend endpoint, it may change based on the namespace you deployed frontend to.
+        prometheus_url: http://frontend.default.svc.cluster.local:9090
+```
+
+**Notes:**
+
+* Authentication is handled automatically via Google Cloud (Workload Identity or default service account in the frontend deployed app)
+* No additional headers or credentials are required
+* The Prometheus Frontend endpoint must be accessible from the cluster
+
 ## Grafana Cloud (Mimir) Configuration
 
 To connect HolmesGPT to Grafana Cloud's Prometheus/Mimir endpoint: