Please take ownership of gogo-genproto files

[kyessenov]

Istio has been maintaining gogo generated files for some opencensus protos: https://github.com/istio/gogo-genproto/tree/master/opencensus/proto

This is becoming a problem since now several downstream dependencies depend on istio to fetch opencensus generated files. Ideally, we want to take istio out of this chain.