Automatic merge from submit-queue (batch tested with PRs 55952, 49112, 55450, 56178, 56151). 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 environment variable to enable support for new Stackdriver resource model
To be merged after #56211
**What this PR does / why we need it**:
This PR adds env variable to control Stackdriver sink in Heapster - whether it export metrics for new resource model or old resource model.
**Release note**:
```release-note
NONE
```
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