Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.

Add etcd DB size monitoring in density test

/cc @wojtek-t 

fyi - @jpbetz @gyuho @kubernetes/sig-scalability-misc 

```release-note
NONE
```