conf(kube-prometheus): Enable all namespaces for ServiceMonitors

Not sure if this is actually needed, but the examples show it included
so doing so.

Author: ssoriche

platform/kube-prometheus/settings.jsonnet

@@ -16,6 +16,7 @@ local kp =
         namespace: 'monitoring',
       },
       prometheus+: {
+        namespaces: [],
         thanos: {
           version: '0.38.0',
           image: 'quay.io/thanos/thanos:v0.30.0',