Sending JSON as Struct from browser (grpc-web) to server (python grpc) results in empty Struct

[Berco Beute]

I've been struggling with passing a JSON with Message values back and forth between a browser (grpc-web) and server (python grpc). A Struct seems to be the way but even though the struct I'm sending looks perfectly fine, it is empty once received by the server.

```

// proto

message Result {

    google.protobuf.Struct variables = 1;

}

// obj - Where variables would contain a (1 level deep) JSON with different types of values, e.g.:

{

    "key1": 1,

    "key2": true,

    "key3": proto_msg_a //instance of proto.MessageA

}

// code (js in browser using grpc-web)

struct = new proto.google.protobuf.Struct(obj);

req = new Request;

req.variables = struct;

```

Checking `req.variables` before sending shows that it's indeed a `Struct` with all the correct fields in it. But once the other end (server) receives it `req.variables` is an empty `Struct`. For testing purposes I tried an `obj` that is simply `{'key': 'value'}`, but the result was the same.

Next thing I tried was `proto.google.protobuf.Struct.fromJavaScript`:

```

// code

struct = proto.google.protobuf.Struct.fromJavaScript(vars);

req = new Request;

req.variables = struct;

```

This works for a simple `obj` (e.g. `{"key": "val"}`), but for an `obj` with a proto message field (such as above) it resulted in :

```

struct_pb.js:875 Uncaught Error: Unexpected struct type.

    at Function.proto.google.protobuf.Value.fromJavaScript (struct_pb.js:875)

    at Function.proto.google.protobuf.Struct.fromJavaScript (struct_pb.js:941)

    at Function.proto.google.protobuf.Value.fromJavaScript (struct_pb.js:871)

    at Function.proto.google.protobuf.Struct.fromJavaScript (struct_pb.js:941)

    at Function.proto.google.protobuf.Value.fromJavaScript (struct_pb.js:871)

    at Function.proto.google.protobuf.Struct.fromJavaScript (struct_pb.js:941)

    at Function.proto.google.protobuf.Value.fromJavaScript (struct_pb.js:871)

    at Function.proto.google.protobuf.Struct.fromJavaScript (struct_pb.js:941)

```

Or should I, instead of going through all the troubles with protobuf/json in javascript, just use a `map`? The nesting in my case will only be 1 level deep anyway. But the values may be anything (proto.MessageX, string, boolean, etc):

```

// proto

message Request {

    map<string, ?type?> variables = 1;

}

```

One advantage is that the receiving end knows all the proto definitions it might receive. But what  would `?typ?` be?

I would really like to use proto messages in variables. The reason for picking protobuf/grpc was exactly this, being able to use the same type throughout our complete platform, but this seems to be blocking this goal. Did I miss something? What would you do?

I've also asked this on StackOverflow: https://stackoverflow.com/questions/62693938/grpc-passing-json-with-proto-message-field-between-browser-and-server