userInfoMiddleware.go

func withUserInfo(handler http.HandlerFunc) http.HandlerFunc {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // look up the value of the Authorization header
        tokenValue := r.Header.Get("Authorization")

        // here is where you would perform some kind of validation on the token
        // but we're going to skip that for this example and save it as the
        // id directly. PLEASE, DO NOT DO THIS IN PRODUCTION.

        // invoke the handler with the new context
        handler.ServeHTTP(w, r.WithContext(
            context.WithValue(r.Context(), "user-id", tokenValue),
        ))
    })
}