Include the target framework in our InstrumentationTests (#7347)

## Summary of changes

Make sure we always specify the target framework when creating a test
console app

## Reason for change

We have some tests where we explicitly create, publish, and run a .NET
Console app. The behaviour we're looking for is dependent on the target
framework, however previously we _weren't_ specifying the TFM to use.
That meant that it was always using the SDK version, even though that is
not expected by the tests (and breaks in some scenarios, e.g. when using
preview SDK builds).

## Implementation details

Explicitly set the target framework to use when creating the console app

## Test coverage

If the tests pass, we're fine.

## Other details

This is based on a bunch of workarounds @bouwkast did in the .NET 10
branch, but they should work standalone too.

As for why we can't just do `dotnet new console -f net6.0`? Who knows😅
for some reason you can't specify an earlier tfm in the .NET template in
our Linux docker images (even though it works on Windows). I'm guessing
it's somehow related to how we install the SDKs, but for now, this is
the hacky workaround

---------

Co-authored-by: Steven Bouwkamp <[email protected]>

Author: andrewlock

tracer/test/Datadog.Trace.ClrProfiler.IntegrationTests/InstrumentationTests.cs

@@ -9,6 +9,7 @@
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text;
+using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using Datadog.Trace.Configuration;
 using Datadog.Trace.TestHelpers;
@@ -83,6 +84,7 @@ public async Task DoesNotInstrumentDotnetBuild()
             using var agent = EnvironmentHelper.GetMockAgent(useTelemetry: true);
 
             var logDir = await RunDotnet("new console -n instrumentation_test -o . --no-restore");
+            FixTfm(workingDir);
             AssertNotInstrumented(agent, logDir);
 
             logDir = await RunDotnet("restore");
@@ -115,6 +117,7 @@ public async Task DoesNotInstrumentExcludedNames(string excludedProcess)
             using var agent = EnvironmentHelper.GetMockAgent(useTelemetry: true);
 
             var logDir = await RunDotnet($"new console -n {excludedProcess} -o . --no-restore");
+            FixTfm(workingDir);
             AssertNotInstrumented(agent, logDir);
 
             var programCs = GetProgramCSThatMakesSpans();
@@ -153,7 +156,9 @@ public async Task DoesInstrumentAllowedProcesses(string allowedProcess)
             using var agent = EnvironmentHelper.GetMockAgent(useTelemetry: true);
 
             var logDir = await RunDotnet($"new console -n {allowedProcess} -o . --no-restore");
+            FixTfm(workingDir);
             AssertNotInstrumented(agent, logDir);
+
             var programCs = GetProgramCSThatMakesSpans();
 
             File.WriteAllText(Path.Combine(workingDir, "Program.cs"), programCs);
@@ -594,7 +599,7 @@ private Task<string> RunDotnetCommand(string workingDirectory, MockTracerAgent m
         {
             // Disable .NET CLI telemetry to prevent extra HTTP spans
             SetEnvironmentVariable("DOTNET_CLI_TELEMETRY_OPTOUT", "1");
-            return RunCommand(workingDirectory, mockTracerAgent, EnvironmentHelper.GetDotnetExe(), arguments);
+            return RunCommand(workingDirectory, mockTracerAgent, "dotnet", arguments);
         }
 
         private async Task<string> RunCommand(string workingDirectory, MockTracerAgent mockTracerAgent, string exe, string arguments = null)
@@ -758,6 +763,28 @@ private bool IsAllLoggingDisabledForBailout()
             return loggingDisabled;
         }
 
+        private void FixTfm(string workingDir)
+        {
+            // This is a hack because for _some_ reason, we can't set the -f flag in the Linux CI.
+            // Force the project to target .NET 8 instead of whatever the SDK defaults to
+            foreach (var projectFile in Directory.GetFiles(workingDir, "*.csproj"))
+            {
+                var projectContent = File.ReadAllText(projectFile);
+
+                // Replace any target framework with the updated version
+                // and add a langversion update to ensure we still compile
+                var replacement = $"""
+                                   <TargetFramework>{EnvironmentHelper.GetTargetFramework()}</TargetFramework>
+                                   <LangVersion>latest</LangVersion>
+                                   """;
+                var updatedContent = Regex.Replace(
+                    projectContent,
+                    @"<TargetFramework>.+</TargetFramework>",
+                    replacement);
+                File.WriteAllText(projectFile, updatedContent);
+            }
+        }
+
         public class TelemetryReporterFixture : IDisposable
         {
             private readonly string _workingDir = Path.Combine(Path.GetTempPath(), Path.GetFileNameWithoutExtension(Path.GetRandomFileName()));