Automatic merge from submit-queue (batch tested with PRs 50223, 53205). 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>.
Create e2e tests for Custom Metrics - Stackdriver Adapter and HPA based on custom metrics from Stackdriver
**What this PR does / why we need it**:
- Add e2e test for Custom Metrics - Stackdriver Adapter
- Add 2e2 test for HPA based on custom metrics from Stackdriver
- Enable HorizontalPodAutoscalerUseRESTClients option
**Release note**:
```release-note
Horizontal pod autoscaler uses REST clients through the kube-aggregator instead of the legacy client through the API server proxy.
```
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