Avoid copying Activity tags and links when sampling

[martincostello]

Do not copy links and tags when creating a new Activity when ActivitySamplingResult.PropagationData is specified.

Fixes #135007.

Benchmarks

BenchmarkDotNet, default job, --inProcess --affinity 4095, creating, starting and stopping an activity with the listener returning the given sampling result. main vs. PR; the tag set is the 9 tags ASP.NET Core-style server instrumentation passes at creation, plus one link. Ratios are PR / main (lower is better).

Sampling	Creation data	Method	main Mean	main Allocated	PR Mean	PR Allocated	Time Ratio	Alloc Ratio
PropagationData	None	NoTags	120.8 ns	416 B	122.0 ns	416 B	1.01	1.00
PropagationData	9 tags	NineTags	211.2 ns	848 B	116.0 ns	416 B	0.55	0.49
PropagationData	9 tags + 1 link	NineTagsAndLink	234.7 ns	976 B	116.0 ns	416 B	0.49	0.43
AllDataAndRecorded	None	NoTags	126.7 ns	416 B	116.4 ns	416 B	0.92	1.00
AllDataAndRecorded	9 tags	NineTags	202.0 ns	848 B	195.0 ns	848 B	0.97	1.00
AllDataAndRecorded	9 tags + 1 link	NineTagsAndLink	242.9 ns	976 B	225.4 ns	976 B	0.93	1.00

Benchmark Code

using System.Diagnostics;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

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

[MemoryDiagnoser]
public class PropagationDataBenchmarks
{
    private static readonly KeyValuePair<string, object?>[] s_tags =
    [
        new("client.address", "192.0.2.1"), new("network.peer.address", "192.0.2.1"), new("network.peer.port", 50000),
        new("server.address", "localhost"), new("server.port", 5000), new("http.request.method", "GET"),
        new("user_agent.original", "Mozilla/5.0"), new("url.scheme", "https"), new("url.path", "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/api/items"),
    ];

    private static readonly ActivityLink[] s_links = [new(new ActivityContext(ActivityTraceId.CreateRandom(), ActivitySpanId.CreateRandom(), ActivityTraceFlags.None))];

    private ActivitySource _source = null!;
    private ActivityListener _listener = null!;
    private ActivitySamplingResult _result;

    [Params(ActivitySamplingResult.PropagationData, ActivitySamplingResult.AllDataAndRecorded)]
    public ActivitySamplingResult Sampling { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        Console.WriteLine("Assembly: " + typeof(Activity).Assembly.Location);
        _result = Sampling;
        _source = new ActivitySource("Bench");
        _listener = new ActivityListener
        {
            ShouldListenTo = s => ReferenceEquals(s, _source),
            Sample = (ref _) => _result,
        };
        ActivitySource.AddActivityListener(_listener);
    }

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

    [Benchmark(Baseline = true)]
    public void NoTags() => Run(null, null);

    [Benchmark]
    public void NineTags() => Run(s_tags, null);

    [Benchmark]
    public void NineTagsAndLink() => Run(s_tags, s_links);

    private void Run(KeyValuePair<string, object?>[]? tags, ActivityLink[]? links)
    {
        var a = _source.CreateActivity("Request", ActivityKind.Server, default(ActivityContext), tags, links);
        a!.Start();
        a.Stop();
    }
}

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[Copilot]

Copilot review overview

🟡 Changes recommended

Sampler tags violate their documented contract, and the behavioral change needs compatibility documentation.

Review effort: Balanced
Findings: 1

Open (1)

• Document missing creation tags and links under PropagationData · New

What changed in this PR

Optimizes Activity creation for PropagationData sampling.

Changes:

• Skips copying creation tags, links, and sampler tags.
• Adds coverage for parent formats and multiple listeners.

File	Description
Activity.cs	Conditionally skips data copying.
ActivitySourceTests.cs	Tests the revised sampling behavior.

[Copilot]

Copilot review overview

🟡 Changes recommended

Public API documentation remains inaccurate for the new sampling semantics.

Review effort: Balanced
Findings: 2

Open (2)

• Update ActivitySource docs for PropagationData tag and link retention · New
• Document missing creation tags and links under PropagationData

[Copilot]

Copilot review overview

🟡 Changes recommended

The existing unresolved compatibility-documentation requirement blocks approval despite the implementation and tests appearing sound.

Review effort: Balanced
Findings: 2

Open (2)

• Update ActivitySource docs for PropagationData tag and link retention
• Document missing creation tags and links under PropagationData

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[tarekgh]

Thanks @martincostello

[github-actions]

Breaking Change Documentation

A breaking change draft has been prepared for this PR.

👉 Click here to create the issue in dotnet/docs

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Note

This documentation was generated with AI assistance from Copilot.

Generated by Breaking Change Documentation · gpt56 · 44 AIC · ⊞ 27.7K · ◷

[tarekgh]

Breaking change issue dotnet/docs#56340 is filed.