Swift Network.framework (websocket): read response headers set by the server

Question

I’m trying to figure it out how a ws client can read additional headers set by the WebSocket server during the handshake.

Websocket Server (built using the NWProtocolWebSocket)

let wsOptions = NWProtocolWebSocket.Options()
wsOptions.setClientRequestHandler(serverQueue) { (_, headers) -> NWProtocolWebSocket.Response in
    let additionalHeaders = [("custom-header", "hi there")]
    return .init(status: .accept, subprotocol: nil, additionalHeaders: additionalHeaders)
}

Websocket Client (built using the NWProtocolWebSocket)

I'm aware that NWProtocolWebSocket.Metadata has an additionalServerHeaders but I don't know how to access it.

Any help? Thanks