Add more explicit logging to ApmAgent.Subscribe. (#2153)

Author: Mpdreamz

src/Elastic.Apm/ApmAgentExtensions.cs

@@ -4,8 +4,10 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Elastic.Apm.Api;
 using Elastic.Apm.DiagnosticSource;
+using Elastic.Apm.Logging;
 
 namespace Elastic.Apm
 {
@@ -29,7 +31,14 @@ public static class ApmAgentExtensions
 		public static IDisposable Subscribe(this IApmAgent agent, params IDiagnosticsSubscriber[] subscribers)
 		{
 			var disposable = new CompositeDisposable();
-			if (!agent.Configuration.Enabled || subscribers is null || subscribers.Length == 0)
+
+			subscribers ??= Array.Empty<IDiagnosticsSubscriber>();
+			var subscribersList = string.Join(", ", subscribers.Select(s => s.GetType().Name));
+
+			agent.Logger.Trace()?.Log("Agent.Subscribe(), Agent Enabled: {AgentEnabled} Subscriber count: {NumberOfSubscribers}, ({Subscribers})",
+				agent.Configuration.Enabled, subscribers.Length, subscribersList);
+
+			if (!agent.Configuration.Enabled || subscribers.Length == 0)
 				return disposable;
 
 			foreach (var subscriber in subscribers)