protoc-gen-go: Add JSON tags to oneof values and members

Oneof values should include json tags to use proper camelCase name and
can be safely omitted when we're handling the interface.

Author: 3v1n0

cmd/protoc-gen-go/internal_gengo/main.go

@@ -378,6 +378,7 @@ func genMessageField(g *protogen.GeneratedFile, f *fileInfo, m *messageInfo, fie
 
 		tags := structTags{
 			{"protobuf_oneof", string(oneof.Desc.Name())},
+			{"json", string(oneof.Desc.JSONName()) + ",omitempty"},
 		}
 		if m.isTracked {
 			tags = append(tags, gotrackTags...)
@@ -812,6 +813,7 @@ func genMessageOneofWrapperTypes(g *protogen.GeneratedFile, f *fileInfo, m *mess
 			goType, _ := fieldGoType(g, f, field)
 			tags := structTags{
 				{"protobuf", fieldProtobufTagValue(field)},
+				{"json", string(field.Desc.JSONName())},
 			}
 			if m.isTracked {
 				tags = append(tags, gotrackTags...)

cmd/protoc-gen-go/testdata/comments/comments.pb.go

@@ -351,6 +351,7 @@ func (od *Oneof) Options() protoreflect.ProtoMessage {
 func (od *Oneof) Fields() protoreflect.FieldDescriptors  { return &od.L1.Fields }
 func (od *Oneof) Format(s fmt.State, r rune)             { descfmt.FormatDesc(s, r, od) }
 func (od *Oneof) ProtoType(protoreflect.OneofDescriptor) {}
+func (od *Oneof) JSONName() string                       { return strs.JSONCamelCase(string(od.Name())) }
 
 type (
 	Extension struct {