Fix exception in Log4NetTracingInterceptor default constructor (#1237)

* Make it possible to instantiate Log4NetTracingInterceptor without configuration file.

* Mention ServiceClientTracing.IsEnabled in README file.

src/client/Microsoft.Rest.ClientRuntime.Log4Net/Log4NetTracingInterceptor.cs

@@ -17,28 +17,39 @@ public class Log4NetTracingInterceptor : IServiceClientTracingInterceptor
     {
         private ILog _logger;
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Log4NetTracingInterceptor" /> class with log4net logger.
+        /// </summary>
+        /// <param name="logger">log4net logger.</param>
+        public Log4NetTracingInterceptor(ILog logger)
+        {
+            _logger = logger;
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="Log4NetTracingInterceptor" /> class with configuration file.
         /// </summary>
         /// <param name="filePath">The configuration file absolute path.</param>
         public Log4NetTracingInterceptor(string filePath)
+            : this(LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType))
         {
-            _logger = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
-
-            if (!string.IsNullOrEmpty(filePath) && File.Exists(filePath))
-            {
-                log4net.Config.XmlConfigurator.ConfigureAndWatch(new FileInfo(filePath));
-            }
-            else
+            if (!string.IsNullOrEmpty(filePath))
             {
-                throw new FileNotFoundException(filePath);
+                if (File.Exists(filePath))
+                {
+                    log4net.Config.XmlConfigurator.ConfigureAndWatch(new FileInfo(filePath));
+                }
+                else
+                {
+                    throw new FileNotFoundException(filePath);
+                }
             }
         }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Log4NetTracingInterceptor" /> class without configuration file.
         /// </summary>
-        public Log4NetTracingInterceptor() : this(null)
+        public Log4NetTracingInterceptor() : this(string.Empty)
         {
         }
 

src/client/Microsoft.Rest.ClientRuntime.Log4Net/README.md

@@ -35,7 +35,8 @@ Using Log4Net for AutoRest Generated Clients:
 ```
 	B) Passing the config file name to ```Log4NetTracingInterceptor``` constructor.
 
-3- Last step is to register the logger into the ServiceClientTracing by having this line called at the start of the application:
+3- Last step is to register the logger into the ServiceClientTracing and enable tracing by having these lines called at the start of the application:
 ```csharp
 	ServiceClientTracing.AddTracingInterceptor(new Log4NetTracingInterceptor());
+	ServiceClientTracing.IsEnabled = true;
 ```