can't collect docker metrics using prometheus

I want to monitor the docker engine. I am following this page Collect Docker metrics with Prometheus to configure the prometheus.

On my PC (OS version is CentOS 7.4, docker version is 17.12.0.ce), I have pasted the following to /etc/docker/daemon.json

  "metrics-addr" : "",
  "experimental" : true

Then, starting the prometheus instance, but i found the connection between prometheus and docker is failed.

Error log:

Get http://localhost:9323/metrics: dial tcp connect: connection refused

What can I do next?