Grafana cannot get the data using rethinkdb_exporter at localhost

Following the dashboard RethinkDB dashboard for Grafana | Grafana Labs to export rethinkdb system stats to localhost:9123
the connection with db is successful.
after importing the dashboard and setting up the data source as Prometheus to http://localhost:9123/metrics, the grafana dashboard can not get the stats under /metrics but keeps get the webpage from localhost:9123, as shown in the screenshot below.

How to resolve this? any ideas?