Update status of label istio.io/rev

[nmittler]

https://github.com/istio/api/blob/master/label/labels.yaml

[ ] Configuration Infrastructure [x] Docs [ ] Installation [ ] Networking [ ] Performance and Scalability [ ] Policies and Telemetry [ ] Security [ ] Test and Release [ ] User Experience

Expected behavior

Steps to reproduce the bug

Version (include the output of istioctl version --remote and kubectl version)

How was Istio installed?

Environment where bug was observed (cloud vendor, OS, etc)