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.
```