Tricky part about implementing that is that the protobuf wire-format alone doesn't contain enough information to unambiguously represent the real data types, it also needs the schema (.proto file) to do that.

For example, in the wire-format, a string and a sub-message are encoded as the same type (a blob - varint + sequence of bytes), but using the schema they are clearly interpreted differently.

Sure, it is possible to make a self-describing protobuf message which includes the schema in protobuf representation, but that is a special-case.