feat(monitoring): stand down Prometheus server during VM trial

vmagent scrapes and remote-writes to VictoriaMetrics, so the Prometheus server scales to 0. Encoded as prometheusSpec.replicas in values instead of a kubectl patch, so helm keeps owning spec.replicas and Helm 4 server-side apply stops conflicting with the kubectl-patch field manager.

prometheus-stack/k8s/grafana-values.yaml

@@ -60,6 +60,10 @@ grafana:
 
 prometheus:
   prometheusSpec:
+    # VM trial: vmagent scrapes and remote-writes to VictoriaMetrics, so the
+    # Prometheus server itself stands down. Encoded here (not a kubectl patch)
+    # so helm keeps owning spec.replicas and upgrades do not conflict on it.
+    replicas: 0
     retention: 60d
     retentionSize: 32GB
     storageSpec: