encodeRequest_(serialized) {
    let len = serialized.length;
    const bytesArray = [0, 0, 0, 0];
    const payload = new Uint8Array(5 + len);
    for (let i = 3; i >= 0; i--) {
      bytesArray[i] = (len % 256);
      len = len >>> 8;
    }
    payload.set(new Uint8Array(bytesArray), 1);
    payload.set(serialized, 5);
    return payload;
  }
Encode the grpc-web request

@private
@param {!Uint8Array} serialized The serialized proto payload
@return {!Uint8Array} The application/grpc-web padded request