https://wiki.jenkins-ci.org/display/JENKINS/Redmine+Plugin
This plugin uses Redmine REST API - to enable it, login into your Redmine, navigate to Administration -> Settings -> Authentication, check "Enable REST web service" and Save.
To use "Aggregate Redmine ticket metrics" you should provide API key assigned to existing Redmine user. Login as desired user, visit http://redmine/my/account, copy key from "API access key" and paste it into job on Jenkins.
Fork and send a pull request (or create an issue on github)