fix(docs): Metric namespace should be optional

Closes: #21430

Signed-off-by: Jesse Szwedko <[email protected]>

website/cue/reference/data_model/schema.cue

@@ -237,7 +237,7 @@ data_model: schema: {
 
 				"namespace": {
 					description: "The metric namespace. Depending on the service, this will prepend the name or use native namespacing facilities."
-					required:    true
+					required:    false
 					type: string: {
 						examples: ["host", "apache", "nginx"]
 					}