examples: adapt examples to stop modifying prometheus object when it is not needed

examples/additional-namespaces-servicemonitor.jsonnet

examples/additional-namespaces.jsonnet

examples/alertmanager-config-external.jsonnet

examples/etcd.jsonnet

examples/kustomize.jsonnet

examples/minikube.jsonnet

examples/prometheus-additional-alert-rule-example.jsonnet

...ples/prometheus-additional-recording-rule-example.jsonnet

examples/prometheus-additional-rendered-rule-example.jsonnet