cue: observe JSON mappings for google.protobuf.Struct

Change-Id: I234767c9bcdb67dee3492827cf598f5a45258959
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2681
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/testdata/attributes.proto.out.cue b/encoding/protobuf/testdata/attributes.proto.out.cue
index 9c13d65..dc5a1b1 100644
--- a/encoding/protobuf/testdata/attributes.proto.out.cue
+++ b/encoding/protobuf/testdata/attributes.proto.out.cue
@@ -19,6 +19,15 @@
 	"github.com/golang/protobuf/ptypes/timestamp"
 )
 
+StructWrap: {
+	struct?:    {}     @protobuf(1,type=google.protobuf.Struct)
+	any?:       _      @protobuf(2,type=google.protobuf.Value)
+	listVal?:   [...]  @protobuf(3,type=google.protobuf.ListValue)
+	boolVal?:   bool   @protobuf(4,type=google.protobuf.BoolValue)
+	stringVal?: string @protobuf(5,type=google.protobuf.StringValue)
+	numberVal?: number @protobuf(6,type=google.protobuf.NumberValue)
+}
+
 //  Attributes represents a set of typed name/value pairs. Many of Mixer's
 //  API either consume and/or return attributes.
 //