Add missing key for DD_TRACE_ACTIVITY_LISTENER

Author: anna-git

tracer/src/Datadog.Trace/Configuration/ConfigurationKeys.cs

@@ -17,6 +17,11 @@ internal static partial class ConfigurationKeys
         // temporary so that ConfigurationKeys2 can resolve reference, all this is removed later
         public const string TraceLogPath = "DD_TRACE_LOG_PATH";
 
+        /// <summary>
+        /// Configuration key to enable or disable the ActivityListener.
+        /// </summary>
+        public const string ActivityListenerEnabled = "DD_TRACE_ACTIVITY_LISTENER_ENABLED";
+
         /// <summary>
         /// Configuration key to enable experimental features.
         /// </summary>