You can access the global variable normalizedPayload which contains the following metadata (https://docs.datacake.de/):

{
    "deveui": string, // The device's DevEUI
    "port": number, // The message's port
    "counter": number,
    "frequency": number, // GHz
    "data_rate": string, // Data rate according to https://blog.dbrgn.ch/2017/6/23/lorawan-data-rates/
    "coding_rate": string,
    "gateways": [
        "id": string,
        "datetime": date,
        "channel": number,
        "rssi": number,
        "snr": number
    ]
}

Decoder example (https://pollutants.eu):

 function Decoder(bytes, port, uplink_info) {
  var decoded = {};
 decoded.PM2_5 = bytes[0]*100 +  bytes[1];
 decoded.T = bytes[2] +  bytes[3]/100;
 decoded.TH = bytes[4] +  bytes[5]/100;
 decoded.RSSI = normalizedPayload.gateways[0].rssi.toFixed(0);
 decoded.SNR = normalizedPayload.gateways[0].snr.toFixed(0);
  return decoded;

 }