[perf] Avoid boxing TagList in Activity.AddException

[martincostello]

Avoid boxing TagList plus an enumerator when Activity.AddException() is used by adding a span-based fast path.

Benchmarks

BenchmarkDotNet v0.15.4, Windows 11 (10.0.26200.9457)
13th Gen Intel Core i7-13700H 2.90GHz, 1 CPU, 20 logical and 14 physical cores
.NET SDK 11.0.100-rc.1.26420.103
  [Host] : .NET 11.0.0 (11.0.0-rc.1.26420.103, 11.0.26.42103), X64 RyuJIT x86-64-v3

Job=MediumRun  Toolchain=InProcessEmitToolchain  IterationCount=15  LaunchCount=2  WarmupCount=10

Build	Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
main	AddException	3.848 us	0.1399 us	0.1961 us	0.3204	0.1068	4.01 KB
PR	AddException	3.712 us	0.2154 us	0.3019 us	0.2975	0.1068	3.69 KB

vs. main: -3.5% time, -8.0% allocated (-320 B)

Benchmark code

using System.Diagnostics;
using BenchmarkDotNet.Attributes;

namespace System.Diagnostics.Microbenchmarks;

[MemoryDiagnoser]
public class ActivityAddExceptionBenchmarks
{
    private ActivitySource _source = null!;
    private ActivityListener _listener = null!;
    private Activity _activity = null!;
    private Exception _exception = null!;

    [GlobalSetup]
    public void Setup()
    {
        _source = new ActivitySource(nameof(ActivityAddExceptionBenchmarks));

        _listener = new ActivityListener
        {
            ShouldListenTo = _ => true,
            Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllData,
        };
        ActivitySource.AddActivityListener(_listener);

        _activity = _source.StartActivity("benchmark-activity")!;
        _exception = new InvalidOperationException("Benchmark exception message");

        // Force the stack trace to be materialized once up front, same as it would be by the time
        // a real exception reaches instrumentation code.
        try
        {
            throw _exception;
        }
        catch (InvalidOperationException)
        {
            // Intentionally swallowed: we only want a populated stack trace for the benchmark.
        }
    }

    [GlobalCleanup]
    public void Cleanup()
    {
        _activity.Dispose();
        _source.Dispose();
        _listener.Dispose();
    }

    [Benchmark]
    public Activity AddException() => _activity.AddException(_exception);
}

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused optimization preserves tag ordering and behavior, retains cross-target compatibility, and is supported by allocation benchmarks and existing tests.

Review effort: Balanced
Findings: None

What changed in this PR

Optimizes Activity.AddException by copying TagList entries directly from a span, avoiding two boxing allocations on modern .NET.

Changes:

• Adds a span-based TagsLinkedList constructor.
• Uses the fast path while preserving the existing fallback for other targets.

File	Description
ActivityEvent.cs	Selects the span-based tag-copy path on .NET.
Activity.cs	Adds direct linked-list construction from a tag span.

[tarekgh]

/ba-g the failure is unrelated

[tarekgh]

LGTM, thanks!