[log4net-bridge] limit chances of duplicated logs export both bridges are enabled

docs/internal/log4net.md

@@ -8,6 +8,9 @@ Both bridge and trace context injection are supported for `log4net` in versions
 
 ## `log4net` [logs bridge](https://opentelemetry.io/docs/specs/otel/glossary/#log-appender--bridge)
 
+If `log4net` is used as a [logging provider](https://learn.microsoft.com/en-us/dotnet/core/extensions/logging-providers), `log4net` bridge should not be enabled, in order
+to reduce possibility of duplicated logs export.
+
 The `log4net` logs bridge is disabled by default. In order to enable it, set `OTEL_DOTNET_AUTO_LOGS_ENABLE_LOG4NET_BRIDGE` to `true`.
 When `log4net` logs bridge is enabled, and `log4net` is configured with at least 1 appender, application logs are exported in OTLP 
 format by default to the local instance of OpenTelemetry Collector, in addition to being written into their currently configured destination (e.g. a file).

src/OpenTelemetry.AutoInstrumentation/Instrumentations/Log4Net/Bridge/Integrations/AppenderCollectionIntegration.cs

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 using OpenTelemetry.AutoInstrumentation.CallTarget;
+using OpenTelemetry.AutoInstrumentation.Logging;
 #if NET
 using OpenTelemetry.AutoInstrumentation.Logger;
 #endif
@@ -23,16 +24,26 @@ namespace OpenTelemetry.AutoInstrumentation.Instrumentations.Log4Net.Bridge.Inte
 type: InstrumentationType.Log)]
 public static class AppenderCollectionIntegration
 {
+    private static readonly IOtelLogger Logger = OtelLogging.GetLogger();
+#if NET
+    private static int _warningLogged;
+#endif
+
     internal static CallTargetReturn<TReturn> OnMethodEnd<TTarget, TReturn>(TTarget instance, TReturn returnValue, Exception exception, in CallTargetState state)
     {
-        if (
-            Instrumentation.LogSettings.Value.EnableLog4NetBridge &&
 #if NET
-#pragma warning disable SA1003
-            !LoggerInitializer.IsInitializedAtLeastOnce &&
-#pragma warning restore SA1003
+        if (LoggerInitializer.IsInitializedAtLeastOnce)
+        {
+            if (Interlocked.Exchange(ref _warningLogged, 1) != default)
+            {
+                return new CallTargetReturn<TReturn>(returnValue);
+            }
+
+            Logger.Warning("Disabling addition of log4net bridge due to ILogger bridge initialization.");
+            return new CallTargetReturn<TReturn>(returnValue);
+        }
 #endif
-            returnValue is Array responseArray)
+        if (Instrumentation.LogSettings.Value.EnableLog4NetBridge && returnValue is Array responseArray)
         {
             var finalArray = OpenTelemetryAppenderInitializer<TReturn>.Initialize(responseArray);
             return new CallTargetReturn<TReturn>(finalArray);

src/OpenTelemetry.AutoInstrumentation/Instrumentations/Log4Net/Bridge/OpenTelemetryLog4NetAppender.cs

@@ -7,6 +7,9 @@
 using System.Reflection;
 using OpenTelemetry.AutoInstrumentation.DuckTyping;
 using OpenTelemetry.AutoInstrumentation.Instrumentations.Log4Net.TraceContextInjection;
+#if NET
+using OpenTelemetry.AutoInstrumentation.Logger;
+#endif
 using OpenTelemetry.AutoInstrumentation.Logging;
 using OpenTelemetry.Logs;
 using Exception = System.Exception;
@@ -28,7 +31,11 @@ internal class OpenTelemetryLog4NetAppender
     private static readonly Lazy<OpenTelemetryLog4NetAppender> InstanceField = new(InitializeAppender, true);
 
     private readonly Func<string?, object?>? _getLoggerFactory;
-    private readonly ConcurrentDictionary<string, object> _loggers = new();
+    private readonly ConcurrentDictionary<string, object> _loggers = new(StringComparer.Ordinal);
+
+#if NET
+    private int _warningLogged;
+#endif
 
     private OpenTelemetryLog4NetAppender(LoggerProvider loggerProvider)
     {
@@ -48,6 +55,19 @@ public void DoAppend(ILoggingEvent loggingEvent)
             return;
         }
 
+#if NET
+        if (LoggerInitializer.IsInitializedAtLeastOnce)
+        {
+            if (Interlocked.Exchange(ref _warningLogged, 1) != default)
+            {
+                return;
+            }
+
+            Logger.Warning("Disabling log4net bridge due to ILogger bridge initialization.");
+            return;
+        }
+#endif
+
         var logger = GetLogger(loggingEvent.LoggerName);
 
         var logEmitter = OpenTelemetryLogHelpers.LogEmitter;

test/IntegrationTests/Log4NetBridgeTests.cs

@@ -111,7 +111,7 @@ public async Task SubmitLogs_ThroughILoggerBridge_WhenLog4NetIsUsedAsILoggerAppe
         using var collector = new MockLogsCollector(Output);
         SetExporter(collector);
 
-        collector.ExpectCollected(records => records.Count == 2, "App logs should be exported once.");
+        collector.ExpectCollected(records => records.Count == 3, "App logs should be exported once.");
 
         EnableBytecodeInstrumentation();
         SetEnvironmentVariable("OTEL_DOTNET_AUTO_LOGS_ENABLE_LOG4NET_BRIDGE", "true");

test/test-applications/integrations/TestApplication.Log4NetBridge/Log4NetLogger.cs

@@ -24,7 +24,7 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except
                 _log.Info(formatter(state, exception));
                 break;
             case LogLevel.Error:
-                _log.Error(formatter(state, exception));
+                _log.Error(formatter(state, exception), exception);
                 break;
             default:
                 throw new ArgumentOutOfRangeException(nameof(logLevel), logLevel, null);

test/test-applications/integrations/TestApplication.Log4NetBridge/Program.cs

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 using System.Diagnostics;
+using log4net;
 using log4net.Config;
 using Microsoft.Extensions.Logging;
 
@@ -39,6 +40,8 @@ private static void Main(string[] args)
 
     private static void LogUsingILogger()
     {
+        var l = LogManager.GetLogger("TestApplication.Log4NetBridge");
+        l.Warn("Before logger factory is built.");
         using var loggerFactory = LoggerFactory.Create(builder =>
         {
             builder.AddProvider(new Log4NetLoggerProvider());