Enabling the Nginx plugin for collectd under CentOS (or any other system using SELinux) might be confusing for a newbie. Most sources on the Internet would just install collectd-nginx:
yum install -y collectd-nginx
and configure it in the nginx.conf and collectd.conf. Still, the statistics might not work as expected, the collectd may not be able to gather statistics from the Nginx.
SELinux may prevent collectd (plugin) daemon to connect to Nginx and gather statistics from the Nginx stats page.
Checking the collectd log and it reports a problem:
Keep on reading!