Avoid allocations propagating Activity baggage

[martincostello]

Avoid allocations when enumerating baggage in internal code paths by using a struct-based enumerator.

Public API surface is unchanged.

Method	Baseline (main) Mean	Fixed Mean	Ratio	Baseline Allocated	Fixed Allocated	Alloc Ratio
GetBaggageItem	17.10 ns	3.37 ns	5.07x	64 B	0 B	0.00x
EnumerateBaggageViaProperty	30.88 ns	30.24 ns	1.02x	64 B	64 B	1.00x

Benchmarks

Before

BenchmarkDotNet v0.14.0, Windows 11 (10.0.26200.9168)
13th Gen Intel Core i7-13700H, 1 CPU, 20 logical and 14 physical cores
.NET SDK 11.0.100-preview.7.26381.103
  [Host] : .NET 10.0.10 (10.0.1026.32716), X64 RyuJIT AVX2

Toolchain=InProcessEmitToolchain

Method	Mean	Error	StdDev	Gen0	Allocated
GetBaggageItem	17.10 ns	0.367 ns	0.477 ns	0.0051	64 B
EnumerateBaggageViaProperty	30.88 ns	0.643 ns	1.343 ns	0.0051	64 B

After

BenchmarkDotNet v0.14.0, Windows 11 (10.0.26200.9168)
13th Gen Intel Core i7-13700H, 1 CPU, 20 logical and 14 physical cores
.NET SDK 11.0.100-preview.7.26381.103
  [Host] : .NET 10.0.10 (10.0.1026.32716), X64 RyuJIT AVX2

Toolchain=InProcessEmitToolchain

Method	Mean	Error	StdDev	Gen0	Allocated
GetBaggageItem	3.370 ns	0.1249 ns	0.1227 ns	-	-
EnumerateBaggageViaProperty	30.241 ns	0.6105 ns	0.6532 ns	0.0051	64 B

using System.Diagnostics;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Jobs;
using BenchmarkDotNet.Running;
using BenchmarkDotNet.Toolchains.InProcess.Emit;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class InProcessConfig : ManualConfig
{
    public InProcessConfig()
    {
        AddJob(Job.Default.WithToolchain(InProcessEmitToolchain.Instance));
        AddDiagnoser(BenchmarkDotNet.Diagnosers.MemoryDiagnoser.Default);
    }
}

[Config(typeof(InProcessConfig))]
public class Bench
{
    private ActivitySource _source = default!;
    private Activity _leaf = default!;

    [GlobalSetup]
    public void Setup()
    {
        _source = new ActivitySource("bench");
        ActivityListener listener = new ActivityListener
        {
            ShouldListenTo = _ => true,
            Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllData,
        };
        ActivitySource.AddActivityListener(listener);

        // Build a chain of 5 activities, baggage set on the root only,
        // so GetBaggageItem/Baggage must walk up the parent chain.
        Activity? root = _source.StartActivity("root");
        root!.SetBaggage("key1", "value1");
        root.SetBaggage("key2", "value2");
        root.SetBaggage("key3", "value3");

        Activity current = root;
        for (int i = 0; i < 4; i++)
        {
            current = _source.StartActivity("child" + i)!;
        }

        _leaf = current;
    }

    [Benchmark]
    public string? GetBaggageItem()
    {
        return _leaf.GetBaggageItem("key3");
    }

    [Benchmark]
    public int EnumerateBaggageViaProperty()
    {
        int count = 0;
        foreach (KeyValuePair<string, string?> kvp in _leaf.Baggage)
        {
            count++;
        }
        return count;
    }
}

[azure-pipelines]

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[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @dotnet/area-system-diagnostics-tracing
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[Copilot]

Pull request overview

This PR reduces allocations in internal baggage-propagation code paths by introducing a struct-based baggage enumerator on Activity and updating propagators to consume it instead of allocating iterator-based enumeration.

Changes:

• Add an internal Activity.EnumerateBaggage() struct enumerator to traverse baggage across an activity’s ancestor chain without iterator allocations.
• Update W3C and legacy/ passthrough propagators to pass the Activity and enumerate baggage via the new struct enumerator.
• Update Activity.GetBaggageItem to use the new enumeration path.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/W3CPropagator.cs	Switch baggage injection to use Activity.EnumerateBaggage() (struct enumerator) to avoid iterator allocations.
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/PassThroughPropagator.cs	Thread root Activity through to baggage injection so it can enumerate baggage allocation-free.
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/LegacyPropagator.cs	Switch baggage injection to pass Activity and enumerate via the struct enumerator.
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DistributedContextPropagator.cs	Update internal baggage injection helper to accept Activity? and use struct-based enumeration.
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs	Introduce EnumerateBaggage() and BaggageEnumerator; update GetBaggageItem to use it.

[azure-pipelines]

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