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", "/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.

[tarekgh]

@martincostello The SamplingTags documentation promises that tags added during sampling are added to a created activity. Could we leave if (samplerTags != null) ungated and update the test to expect those tags under PropagationData?

This would not address the separate compatibility concern for creation tags and links. We should weigh that observable change against the measured benefit and document it if we proceed.

[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

[tarekgh]

The documentation concern is valid. Here is suggested replacement wording for the tags and links parameter documentation in ActivitySource.cs, to apply consistently to the two CreateActivity overloads and three StartActivity overloads that accept those parameters.

        /// <param name="tags">
        /// The optional tags made available to sampling callbacks and copied to the created activity
        /// only when the combined sampling result is <see cref="ActivitySamplingResult.AllData"/>
        /// or <see cref="ActivitySamplingResult.AllDataAndRecorded"/>.
        /// </param>
        /// <param name="links">
        /// The optional links made available to sampling callbacks and copied to the created activity
        /// only when the combined sampling result is <see cref="ActivitySamplingResult.AllData"/>
        /// or <see cref="ActivitySamplingResult.AllDataAndRecorded"/>.
        /// </param>

This wording applies only to caller-supplied creation tags and links. Tags added through ActivityCreationOptions.SamplingTags are still copied when the combined result is PropagationData.

The block above is for manual application to ActivitySource.cs, not to the Activity.cs lines attached to this thread. ActivitySource.cs is not currently part of the PR diff, so this is not a directly applicable suggestion here.

[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