fix #79 only inject layout renderer keys for APM if Elastic.Apm.NLog … (

#83)

* fix #79 only inject layout renderer keys for APM if Elastic.Apm.NLog is referenced

* move logic to class instantation

src/Elastic.CommonSchema.NLog/EcsLayout.cs

@@ -6,6 +6,8 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
+using System.Reflection;
 using System.Text;
 using NLog;
 using NLog.Config;
@@ -21,6 +23,15 @@ public class EcsLayout : Layout
 	{
 		public const string Name = nameof(EcsLayout);
 
+		private static bool? _nlogApmLoaded;
+
+		private static bool NLogApmLoaded()
+		{
+			if (_nlogApmLoaded.HasValue) return _nlogApmLoaded.Value;
+			_nlogApmLoaded = Type.GetType("Elastic.Apm.NLog.ApmTraceIdLayoutRenderer, Elastic.Apm.NLog") != null;
+			return _nlogApmLoaded.Value;
+		}
+
 		private readonly Layout _disableThreadAgnostic = "${threadid:cached=true}";
 
 		public EcsLayout()
@@ -42,8 +53,11 @@ public EcsLayout()
 			ServerUser = "${environment-user}"; // NLog 4.6.4
 
 			// These values are set by the Elastic.Apm.NLog package
-			ApmTraceId = "${ElasticApmTraceId}";
-			ApmTransactionId = "${ElasticApmTransactionId}";
+			if (NLogApmLoaded())
+			{
+				ApmTraceId = "${ElasticApmTraceId}";
+				ApmTransactionId = "${ElasticApmTransactionId}";
+			}
 		}
 
 		public Layout AgentId { get; set; }