Custom http header not getting push

Question

I am setting a custom http header on bramble a federated graphql gateway server, but it didn't push to my federated service or maybe I'm doing it wrong. If anyone encounter this same issue please educate me, thanks. I using gin for my web framework.

Here's what I done so far,

Im setting this header when calling the api through the gateway server

{
   "X-Custom-Header":"SomeValue"
}

and get that from the request

func graphHandler() gin.HandlerFunc {

  return func(c *gin.Context) {
      v := c.Request.Header.Get("X-Custom-Header")
      log.Println("X-Custom-Header value", v)
    
      config := graph.Config{Resolvers: &graph.Resolver{}}
      config.Directives.Boundary = func(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
        return next(ctx)
      }

    h := handler.NewDefaultServer(graph.NewExecutableSchema(config))
    h.ServeHTTP(c.Writer, c.Request)
  }
}

func main() {

  godotenv.Load()

  port := os.Getenv("PORT")
  if port == "" {
      port = defaultPort
  }

  r := gin.Default()

  r.POST("/query", graphHandler())
  r.GET("/", playgroundHandler())

  r.Run(":" + port)
}

gateway configuration (config.json)

{
  "services": [
    "http://app1:8080/query",
    "http://user:8080/query"
  ],
  "gateway-port": 8082,
  "private-port": 8083,
  "metrics-port": 8084,
  "log-level": "debug",
  "poll-interval": "5s",
  "max-requests-per-query": 50,
  "max-client-response-size": 1048576,
  "id-field-name": "id",
  "plugins":[
    {
      "name": "cors",
        "config": {
        "allowed-origins": ["*"],
        "allowed-headers": ["x-custom-header"],
        "allow-credentials": true,
        "max-age": 3600,
        "debug": true
      }
    },
    {
      "name": "admin-ui"
    },
    {
      "name": "playground"
    }  
  ]
}