Update Redis dashboard (DataDog#22301)

* Update Redis dashboard

* Fix unit for slowlog duration

* Minor changes

* Fix diff on rejected connections

* Add images

* Improve images

* Improve key length widget

* Specify theme in image caption

* Add diff formula to rejected connections and display key lengths as distribution

* Update screenshots

* Fix CI

* Attempt to fix logs validations

* Revert "Attempt to fix logs validations"

This reverts commit 94bc138.

Author: dkirov-dd