added back MqttTrace consuming api

MQTTnet.Core/Diagnostics/MqttNetTrace.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging;
+
+namespace MQTTnet.Core.Diagnostics
+{
+    public class MqttNetTrace : ILoggerProvider
+    {
+        private readonly ConcurrentDictionary<string, MqttNetLogger> _loggers = new ConcurrentDictionary<string, MqttNetLogger>();
+
+        public static event EventHandler<MqttNetTraceMessagePublishedEventArgs> TraceMessagePublished;
+
+        public void Publish(MqttNetTraceMessage msg)
+        {
+            TraceMessagePublished?.Invoke(this, new MqttNetTraceMessagePublishedEventArgs(msg));
+        }
+
+        public void Dispose()
+        {
+            TraceMessagePublished = null;
+        }
+
+        public ILogger CreateLogger(string categoryName)
+        {
+            return _loggers.GetOrAdd(categoryName, CreateLoggerImplementation);
+        }
+
+        private MqttNetLogger CreateLoggerImplementation(string categoryName)
+        {
+            return new MqttNetLogger(categoryName, this);
+        }
+    }
+}