Ensure isEnabled = false won't start worker loop (#2436)

Inside PayloadSenderV2, we also now early exit when events get queued.

Removed early exit from constructor, allow readonly fields to be
initialized
so that when we start adding nullable annotations not all of them have
to be nullable

src/Elastic.Apm/Report/PayloadSenderV2.cs

@@ -58,6 +58,7 @@ internal class PayloadSenderV2 : BackendCommComponentBase, IPayloadSender, IPayl
 
 		private readonly ElasticVersion _brokenActivationMethodVersion;
 		private readonly string _cachedActivationMethod;
+		private readonly bool _isEnabled;
 
 		public PayloadSenderV2(
 			IApmLogger logger,
@@ -73,27 +74,30 @@ public PayloadSenderV2(
 		)
 			: base(isEnabled, logger, ThisClassName, service, configuration, httpMessageHandler)
 		{
-			if (!isEnabled)
-				return;
-
 			_logger = logger?.Scoped(ThisClassName + (dbgName == null ? "" : $" (dbgName: `{dbgName}')"));
+			_isEnabled = isEnabled;
+			if (!_isEnabled)
+				_logger?.Debug()?.Log($"{nameof(PayloadSenderV2)} is not enabled, work loop won't be started.");
+
 			_payloadItemSerializer = new PayloadItemSerializer();
 			_configuration = configuration;
 			_brokenActivationMethodVersion = new ElasticVersion(8, 7, 0);
-
 			_intakeV2EventsAbsoluteUrl = BackendCommUtils.ApmServerEndpoints.BuildIntakeV2EventsAbsoluteUrl(configuration.ServerUrl);
 
 			System = system;
 
 			_cloudMetadataProviderCollection = new CloudMetadataProviderCollection(configuration.CloudProvider, _logger, environmentVariables);
 			_apmServerInfo = apmServerInfo ?? new ApmServerInfo();
 			_serverInfoCallback = serverInfoCallback;
+
 			var process = ProcessInformation.Create();
 			_metadata = new Metadata { Service = service, System = System, Process = process };
 			foreach (var globalLabelKeyValue in configuration.GlobalLabels)
 				_metadata.Labels.Add(globalLabelKeyValue.Key, globalLabelKeyValue.Value);
 			_cachedActivationMethod = _metadata.Service?.Agent.ActivationMethod;
-			ResetActivationMethodIfKnownBrokenApmServer();
+
+			if (_isEnabled)
+				ResetActivationMethodIfKnownBrokenApmServer();
 
 			if (configuration.MaxQueueEventCount < configuration.MaxBatchEventCount)
 			{
@@ -123,7 +127,9 @@ public PayloadSenderV2(
 			_eventQueue = new BatchBlock<object>(configuration.MaxBatchEventCount);
 
 			SetUpFilters(TransactionFilters, SpanFilters, ErrorFilters, apmServerInfo, logger);
-			StartWorkLoop();
+
+			if (_isEnabled)
+				StartWorkLoop();
 		}
 
 		internal static void SetUpFilters(
@@ -175,6 +181,9 @@ protected override void Dispose(bool disposing)
 
 		internal async Task<bool> EnqueueEventInternal(object eventObj, string dbgEventKind)
 		{
+			if (!_isEnabled)
+				return true;
+
 			// Enforce _maxQueueEventCount manually instead of using BatchBlock's BoundedCapacity
 			// because of the issue of Post returning false when TriggerBatch is in progress. For more details see
 			// https://stackoverflow.com/questions/35626955/unexpected-behaviour-tpl-dataflow-batchblock-rejects-items-while-triggerbatch
@@ -217,8 +226,12 @@ internal async Task<bool> EnqueueEventInternal(object eventObj, string dbgEventK
 			return true;
 		}
 
-		internal void EnqueueEvent(object eventObj, string dbgEventKind) =>
+		internal void EnqueueEvent(object eventObj, string dbgEventKind)
+		{
+			if (!_isEnabled)
+				return;
 			Task.Run(async () => await EnqueueEventInternal(eventObj, dbgEventKind));
+		}
 
 		/// <summary>
 		/// Runs on the background thread dedicated to sending data to APM Server. It's ok to block this thread.