// так используем
var trafficItems = instance.ActiveTab.GetTraffic();
var trafficModel = new TrafficModel(trafficItems);
string serializedTraffic = Global.ZennoLab.Json.JsonConvert.SerializeObject(trafficModel);
File.WriteAllText(@"C:\tmp\traffic.har", serializedTraffic);

// остальное в общий код
[Serializable]
    public class TrafficModel
    {
        public TrafficModel(IEnumerable<TrafficItem> trafficItems)
        {
            log = new ModelLog(trafficItems);
        }

        public ModelLog log;

        [Serializable]
        public class ModelLog
        {
            public ModelLog(IEnumerable<TrafficItem> trafficItems)
            {
                foreach (var trafficItem in trafficItems)
                {
                    entries.Add(new ModelEntry(trafficItem));
                }
            }

            public List<ModelEntry> entries = new List<ModelEntry>();

            [Serializable]
            public class ModelEntry
            {
                public ModelEntry(TrafficItem trafficItem)
                {
                    time = trafficItem.Time;
                    request = new ModelRequest(trafficItem);
                    response = new ModelResponse(trafficItem);
                }

                public int time;
                public ModelRequest request;
                public ModelResponse response;
            }

            [Serializable]
            public class ModelRequest
            {
                public ModelRequest(TrafficItem trafficItem)
                {
                    method = trafficItem.Method;
                    url = trafficItem.Url;
                    cookies = ModelJsonKeyValue.FromString(trafficItem.RequestCookies, ";", "=");
                    headers = ModelJsonKeyValue.FromString(trafficItem.RequestHeaders, "\n", ":");
                    queryString = ModelJsonKeyValue.FromString(trafficItem.RequestQuery, "&", "=");
                }

                public string method;
                public string url;
                public List<ModelJsonKeyValue> cookies;
                public List<ModelJsonKeyValue> headers;
                public List<ModelJsonKeyValue> queryString;
            }

            [Serializable]
            public class ModelResponse
            {
                public ModelResponse(TrafficItem trafficItem)
                {
                    status = trafficItem.ResultCode;
                    statusText = status == 200 ? "OK" : "not ok";
                    cookies = ModelJsonKeyValue.FromString(trafficItem.ResponseCookies, ";", "=");
                    headers = ModelJsonKeyValue.FromString(trafficItem.ResponseHeaders, "\n", ":");
                    content = new ModelContent(trafficItem);

                }

                public uint status;
                public string statusText;
                public List<ModelJsonKeyValue> cookies;
                public List<ModelJsonKeyValue> headers;
                public ModelContent content;

                [Serializable]
                public class ModelContent
                {
                    public ModelContent(TrafficItem trafficItem)
                    {
                        mimeType = trafficItem.ResponseContentType;
                        text = Encoding.UTF8.GetString(trafficItem.ResponseBody ?? new byte[]{});
                    }

                    public string mimeType;
                    public string text;
                }
            }
        }
    }

    public class ModelJsonKeyValue
    {
        public ModelJsonKeyValue(string name, string value)
        {
            this.name = name;
            this.value = value;
        }

        public string name;
        public string value;

        public static List<ModelJsonKeyValue> FromString(string keyValuesString, string pairSeparator, string keyValueSeparator)
        {

            return keyValuesString
                .Split(new []{pairSeparator}, StringSplitOptions.RemoveEmptyEntries)
                .Select(p => KeyValueFromString(p, keyValueSeparator))
                .ToList();


        }

        public static ModelJsonKeyValue KeyValueFromString(string keyValueString, string keyValueSeparator)
            {
                string[] splitted = keyValueString
                    .Split(new[] {keyValueSeparator}, 2, StringSplitOptions.None);

                string key   = splitted.Length >= 1 ? splitted[0] : "";
                string value = splitted.Length >= 2 ? splitted[1] : "";

                return new ModelJsonKeyValue(key, value);
            }
    }