Builds the plain object that represents this message on the wire. Standard fields are included when set, and custom fields are flattened to the top level. This method is invoked automatically by JSON.stringify.
When a new standard field is added to this class, it should be added here as well so that it is included in the serialized request body.
Staticbuild
The model for a request message in the HTTP body that is sent to the server beginning in 4.0.0.