fix: prometheus enabled condition

roles/custom/matrix-prometheus-nginxlog-exporter/tasks/validate_config.yml

@@ -3,8 +3,9 @@
 - name: Fail if Prometheus not enabled
   ansible.builtin.fail:
     msg: >
-      You need to enable `matrix_prometheus_enabled` as this roles requires Prometheus for data storage.
-  when: "not matrix_prometheus_enabled"
+      You need to set `matrix_prometheus_enabled: true` or configure an external Prometheus database
+      as this roles requires Prometheus for data storage.
+  when: not matrix_prometheus_enabled and not matrix_prometheus_nginxlog_exporter_metrics_proxying_enabled
 
 - name: Fail if docker image not availble for arch
   ansible.builtin.fail: