The Pubsub Server Info plugin provides a way for Openfire to report statistics about itself in a well-known pub-sub node: 'serverinfo'. The data format is defined in XEP-0485: PubSub Server Info.
Copy pubsubserverinfo.jar into the plugins directory of your Openfire installation. The plugin will then be automatically deployed. To upgrade to a new version, copy the new pubsubserverinfo.jar file over the existing file.