Adds 'redis.expires.percent' to redis checks

Allows tracking the percentage difference between keys and expired keys so that you can create alerts on that value in Datadog.

This is a direct copy from the work done by @wisq in our internal cookbooks, https://github.com/Shopify/cookbooks/pull/2871

For review, @wisq @skingry 

If it looks okay we can open a PR with DD.