What is the correct Protobuf content type?
JSON has application/json as a standard. For protobuf some people use application/x-protobuf, but I saw something as odd as application/vnd.google.protobuf being proposed. Do we have an RFC or some other standard that I can use as a reference for this?
There's an expired IETF proposal that suggests application/protobuf. It does not address the question how the receiving side could determine the particular message type. Previous discussions suggested using a parameter to specify package and message, e.g. application/protobuf; proto=org.some.Message
In practice, the types you listed seem to be indeed the ones in use, for example the monitoring system Prometheus uses application/vnd.google.protobuf, and the Charles web debugging proxy recognizes application/x-protobuf; messageType="x.y.Z".