Add nsMetadata example to template

templates/default/namespaces.nix

@@ -0,0 +1,28 @@
+# Assign extra metadata here. For example,
+# `default = {labels."istio.io/rev" = "stable"}`
+# is the equivalent of
+# `k label ns/default istio.io/rev=stable`
+rec {
+  # All-caps names are safe to use for special values;
+  # no Kubernetes object can have capital letters.
+  DEFAULT = {
+    labels = {
+      "istio.io/rev" = "stable";
+    };
+  };
+
+  # Opt a namespace out of the defaults.
+  gateway-system = {};
+  kube-system = {};
+  longhorn-system = {};
+
+  # To set data beyond the defaults,
+  # opt the namespace back in.
+  default =
+    DEFAULT
+    // {
+      labels = {
+        "words-words-words-words" = "punchline";
+      };
+    };
+}