GelfJsonFormatter.cs

    public sealed class GelfJsonFormatter : ITextFormatter
    {
        private static readonly IReadOnlyDictionary<LogEventLevel, short> Levels
            = new Dictionary<LogEventLevel, short>()
            {
                { LogEventLevel.Verbose, 7 },
                { LogEventLevel.Debug, 6 },
                { LogEventLevel.Information, 5 },
                { LogEventLevel.Warning, 4 },
                { LogEventLevel.Error, 3 },
                { LogEventLevel.Fatal, 0 },
            };

        public void Format(LogEvent logEvent, TextWriter output)
        {
            using (var json = new JsonTextWriter(output)
            {
                CloseOutput = false,
                Formatting =  Formatting.None
            })
            {
                json.WriteStartObject();

                json.WritePropertyName("version");
                json.WriteValue("1.1");

                json.WritePropertyName("host");
                json.WriteValue(Environment.MachineName);

                json.WritePropertyName("short_message");
                json.WriteValue(logEvent.RenderMessage());

                json.WritePropertyName("timestamp");
                json.WriteValue(logEvent.Timestamp.ToUnixTimeSeconds());

                json.WritePropertyName("level");
                json.WriteValue(Levels[logEvent.Level]);

                if (logEvent.Exception != null)
                {
                    json.WritePropertyName("_error");
                    json.WriteValue(logEvent.Exception.Message);

                    json.WritePropertyName("_error_details");
                    json.WriteValue(logEvent.Exception.ToString());
                }

                foreach (var (key, value) in logEvent.Properties)
                {
                    json.WritePropertyName($"_{key}");
                    json.WriteValue(value.ToString());
                }

                json.WriteEndObject();

            }

            output.WriteLine();
        }
    }