public KubernetesEndpointGroupBuilder namespace(String namespace) {
        this.namespace = requireNonNull(namespace, "namespace");
        return this;
    }
Sets the <a href="https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/">namespace</a>
of a Kubernetes cluster.
kubernetes/src/main/java/com/linecorp/armeria/client/kubernetes/endpoints/KubernetesEndpointGroupBuilder.java