Change promtail labels to arg

manifests/monitoring/promtail.yaml

@@ -56,8 +56,6 @@ data:
       batchwait: 1s
       external_labels: {}
       timeout: 10s
-      external_labels:
-        kubernetes_cluster: scaleway
     positions:
       filename: /run/promtail/positions.yaml
     server:
@@ -399,6 +397,7 @@ spec:
           args:
             - "-config.file=/etc/promtail/promtail.yaml"
             - "-client.url=http://loki.inlets.svc:80/loki/api/v1/push"
+            - "-client.external-labels=kubernetes_cluster=scaleway"
           volumeMounts:
             - name: config
               mountPath: /etc/promtail