Automatic merge from submit-queue (batch tested with PRs 56858, 57040, 56979, 57051, 57052). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add e2e test for custom metrics with Prometheus and Stackdriver

**What this PR does / why we need it**:
Add e2e test for custom metrics in a setup with a container exposing metrics in Prometheus format and prometheus-to-sd sidecar container scraping the metrics and writing them to Stackdriver.
Also some test refactoring for adding new tests more easily (plan to add scale down tests and a test with scaling on two metrics).

**Release note**:
```
NONE
```