added back MqttTrace consuming api

MQTTnet.Core/Diagnostics/MqttNetLogger.cs

@@ -0,0 +1,49 @@
+using System;
+using Microsoft.Extensions.Logging;
+
+namespace MQTTnet.Core.Diagnostics
+{
+    public class MqttNetLogger : ILogger
+    {
+        private readonly string _categoryName;
+        private readonly MqttNetTrace _mqttNetTrace;
+
+        public MqttNetLogger(string categoryName, MqttNetTrace mqttNetTrace)
+        {
+            _categoryName = categoryName;
+            _mqttNetTrace = mqttNetTrace;
+        }
+
+        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
+        {
+            if (formatter == null)
+            {
+                throw new ArgumentNullException(nameof(formatter));
+            }
+
+            var message = formatter(state, exception);
+
+            var traceMessage = new MqttNetTraceMessage(DateTime.Now, Environment.CurrentManagedThreadId, _categoryName, logLevel, message, exception);
+            _mqttNetTrace.Publish(traceMessage);
+        }
+
+        public bool IsEnabled(LogLevel logLevel)
+        {
+            return true;
+        }
+
+        //not supported: async local requires netstandard1.3
+        //for implementation see https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Extensions.Logging.Console/ConsoleLogScope.cs
+        public IDisposable BeginScope<TState>(TState state)
+        {
+            return new DisposableScope();
+        }
+
+        private class DisposableScope : IDisposable
+        {
+            public void Dispose()
+            {
+            }
+        }
+    }
+}