fix(cilium): wire cilium_enable_prometheus to Helm values template

The cilium_enable_prometheus variable was defined in defaults and
documented in sample inventory but never mapped to the Helm values
template, making it a no-op. Add the prometheus.enabled field to
values.yaml.j2 so that setting cilium_enable_prometheus: true
correctly enables Prometheus metrics on the Cilium agent.

Signed-off-by: Kay Yan <kay.yan@daocloud.io>

roles/network_plugin/cilium/templates/values.yaml.j2

@@ -177,6 +177,9 @@ hostFirewall:
 
 policyAuditMode: {{ cilium_policy_audit_mode | to_json }}
 
+prometheus:
+  enabled: {{ cilium_enable_prometheus | to_json }}
+
 certgen:
   image:
     repository: {{ cilium_hubble_certgen_image_repo }}