Correct PROMETHEUS_EXTERNAL_URL - align with Alertmanager

Change-Id: I50efd2620d1e88ecaa99856dc879228771aa0ad4
Related-bug: PROD-34380
diff --git a/docker/swarm/stack/monitoring/prometheus/init.yml b/docker/swarm/stack/monitoring/prometheus/init.yml
index d7db52c..d38f5f8 100644
--- a/docker/swarm/stack/monitoring/prometheus/init.yml
+++ b/docker/swarm/stack/monitoring/prometheus/init.yml
@@ -2,6 +2,8 @@
 - system.prometheus.server.container
 parameters:
   _param:
+    prometheus_external_url: ${_param:cluster_public_host}
+    prometheus_external_proto: https
     # Backward compatibility for Prometheus 1.7
     prometheus_storage_local_engine: "persisted"
     prometheus_storage_heap_size: 3221225472
@@ -39,7 +41,7 @@
                 PROMETHEUS_BIND_PORT: ${prometheus:server:bind:port}
                 PROMETHEUS_BIND_ADDRESS: ${prometheus:server:bind:address}
                 PROMETHEUS_STORAGE_LOCAL_RETENTION: ${prometheus:server:storage:local:retention}
-                PROMETHEUS_EXTERNAL_URL: "https://${_param:stacklight_monitor_address}:15010"
+                PROMETHEUS_EXTERNAL_URL: "${_param:prometheus_external_proto}://${_param:prometheus_external_url}:15010"
                 # Backward compatibility for Prometheus 1.7
                 PROMETHEUS_STORAGE_LOCAL_ENGINE: ${_param:prometheus_storage_local_engine}
                 PROMETHEUS_STORAGE_LOCAL_TARGET_HEAP_SIZE: ${_param:prometheus_storage_heap_size}