[gRPC-gateway] Empty arrays are decoded as {} (which should be [])

[StarlightIbuki]

Is there an existing issue for this?

• I have searched the existing issues

Kong version ($ kong version)

*

Current Behavior

When a gRPC service returns an empty array, it's encoded as {} in the JSON body.

Expected Behavior

It should be encoded as [].

Steps To Reproduce

//... some service
  rpc Echo(EchoMsg) returns (EchoMsg) {
    option (google.api.http) = {
      post: "/v1/echo"
      body: "*"
    };
  }

//...
message EchoMsg {
  repeated string array = 1;
  string nullable = 2;
}

func (s *server) Echo(ctx context.Context, in *pb.EchoMsg) (*pb.EchoMsg, error) {
	return in, nil
}

local req_body = { array = {}, nullable = "ahaha" }
local res, _ = proxy_client:post("/v1/echo", {
   headers = { ["Content-Type"] = "application/json" },
    body = req_body,
})

res:read_body() would return something like { "array": {}, "nullable": "ahaha"}.

Anything else?

This issue is created for visibility for an internal ticket(FTI-5002).