Automatic merge from submit-queue (batch tested with PRs 51824, 50476, 52451, 52009, 52237)
Allow metadata firewall & proxy on in GCE, off by default
**What this PR does / why we need it**: Add necessary variables in kube-env to allow a user to turn on metadata firewall and proxy for K8s on GCE.
Ref #8867.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*:
**Special notes for your reviewer**:
**Release note**:
```release-note
GCE users can enable the metadata firewall and metadata proxy with KUBE_FIREWALL_METADATA_SERVER and ENABLE_METADATA_PROXY, respectively.
```
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