title: grafana label_values show 404
body: <p>I change the default datasouce
  prometheus to ambari
  then all variables which used label_values show 404
  <a href="https://i.stack.imgur.com/bFw8V.png" rel="nofollow noreferrer">enter image description here</a></p>
  <p>now I change default datasource ambari to prometheus,It's not ok</p>