added back MqttTrace consuming api

MQTTnet.Core/Diagnostics/MqttTraceMessage.cs

@@ -0,0 +1,30 @@
+using System;
+using Microsoft.Extensions.Logging;
+
+namespace MQTTnet.Core.Diagnostics
+{
+    public sealed class MqttNetTraceMessage
+    {
+        public MqttNetTraceMessage(DateTime timestamp, int threadId, string source, LogLevel level, string message, Exception exception)
+        {
+            Timestamp = timestamp;
+            ThreadId = threadId;
+            Source = source;
+            Level = level;
+            Message = message;
+            Exception = exception;
+        }
+
+        public DateTime Timestamp { get; }
+
+        public int ThreadId { get; }
+
+        public string Source { get; }
+
+        public LogLevel Level { get; }
+
+        public string Message { get; }
+
+        public Exception Exception { get; }
+    }
+}