Add MQTTnet debug logging.

Source/MQTTnet.Server/Mqtt/CustomMqttFactory.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using Microsoft.Extensions.Logging;
 using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
+using MQTTnet.Server.Configuration;
 using MQTTnet.Server.Logging;
 
 namespace MQTTnet.Server.Mqtt
@@ -11,13 +12,28 @@ namespace MQTTnet.Server.Mqtt
     {
         private readonly MqttFactory _mqttFactory;
 
-        public CustomMqttFactory(ILogger<MqttServer> logger)
+        public CustomMqttFactory(SettingsModel settings, ILogger<MqttServer> logger)
         {
+            if (settings == null) throw new ArgumentNullException(nameof(settings));
             if (logger == null) throw new ArgumentNullException(nameof(logger));
+            
+            // It is important to avoid injecting the logger wrapper to ensure that no
+            // unused log messages are generated by the MQTTnet library. Debug logging
+            // has a huge performance impact.
+            if (settings.EnableDebugLogging)
+            {
+                var mqttNetLogger = new MqttNetLoggerWrapper(logger);
+                _mqttFactory = new MqttFactory(mqttNetLogger);
 
-            Logger = new MqttNetLoggerWrapper(logger);
+                logger.LogWarning("Debug logging is enabled. Performance of MQTTnet Server is decreased!");
+            }
+            else
+            {
+                _mqttFactory = new MqttFactory();
+            }
+
+            Logger = _mqttFactory.DefaultLogger;
 
-            _mqttFactory = new MqttFactory(Logger);
         }
         
         public IMqttNetLogger Logger { get; }