fix: Stop double injecting headers with HttpClient on .NET Framework (#1679)

Author: nrcventura

src/Agent/NewRelic/Agent/Extensions/Providers/Wrapper/HttpClient/SendAsync.cs

@@ -61,7 +61,9 @@ public AfterWrappedMethodDelegate BeforeWrappedMethod(InstrumentedMethodCall ins
 
             var externalSegmentData = transactionExperimental.CreateExternalSegmentData(uri, method);
             var segment = transactionExperimental.StartSegment(instrumentedMethodCall.MethodCall);
-            segment.GetExperimentalApi().SetSegmentData(externalSegmentData);
+            segment.GetExperimentalApi()
+                .SetSegmentData(externalSegmentData)
+                .MakeLeaf();
 
             if (agent.Configuration.ForceSynchronousTimingCalculationHttpClient)
             {

src/Agent/NewRelic/Agent/Extensions/Providers/Wrapper/HttpWebRequest/SerializeHeadersWrapper.cs

@@ -2,7 +2,6 @@
 // SPDX-License-Identifier: Apache-2.0
 
 using System;
-using System.Linq;
 using NewRelic.Agent.Api;
 using NewRelic.Agent.Extensions.Providers.Wrapper;
 
@@ -33,6 +32,16 @@ public AfterWrappedMethodDelegate BeforeWrappedMethod(InstrumentedMethodCall ins
                 throw new NullReferenceException("request.Headers");
             }
 
+            // We should only inject headers with this instrumentation if HttpWebRequest created an external segment.
+            // Both HttpClient and RestSharp instrumentation have their own header injection support and do not rely
+            // on this instrumentation to inject the necessary headers. Other instrumentation rely on leaf segments
+            // to prevent this instrumentation from running.
+            if (!transaction.CurrentSegment.IsExternal)
+            {
+                transaction.LogFinest("Skipping HttpWebRequest header injection because the current segment was not created by HttpWebRequest.");
+                return Delegates.NoOp;
+            }
+
             var setHeaders = new Action<System.Net.HttpWebRequest, string, string>((carrier, key, value) =>
             {
                 carrier.Headers?.Set(key, value);