Implemented optional (presented) fields for proto3

[biosvs]

Ofc this PR will never be merged.

Although if you stuck with gogo, need "presence" feature, and brave enough..

(Added some fields to descriptor.proto manually. Didn't fix some tests - they are broken due to protoc version upgrade; didn't add new tests.)

fix #713

[nightwolfz]

Thanks

[bwplotka]

Thanks!

~We consider using this until we get migrate out of gogoproto in Prometheus~ EDIT: Actually we decided to skip optional, it's too dangerous, even mentioned in the official Google Cloud docs 🙈