Issue
You need to download a dump of your Redis data to analyze usage or perform a backup.
Resolution
We don't currently offer the facility to download rdb dumps (https://devcenter.heroku.com/articles/heroku-redis#persistence). For analysis another option is to try forking the Redis addon. You can find an example of the command for this here: https://devcenter.heroku.com/articles/heroku-redis#migrating-to-heroku-redis
With the fork in place this should allow you to use third party analysis tools or external replication without affecting the production instance.