[perf] Add fast path for unescaped W3C baggage

[martincostello]

Mirror EncodeBaggageValue() by adding a fast path for baggage values that do not need percent decoding.

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	Allocated
main	ExtractPlainBaggage	160.0 ns	12.69 ns	19.00 ns	0.0293	368 B
PR	ExtractPlainBaggage	145.0 ns	8.16 ns	11.97 ns	0.0293	368 B
main	ExtractEscapedBaggage	176.6 ns	12.07 ns	17.69 ns	0.0279	352 B
PR	ExtractEscapedBaggage	178.3 ns	13.16 ns	19.29 ns	0.0279	352 B

vs. main: plain (common case) ~9% faster, allocation unchanged (a string must be produced
either way). Escaped (uncommon case): within noise across repeated runs (bounced between -4%
and +3%), versus a consistent ~+5% before this change.

Benchmark code

using System.Diagnostics;
using System.Linq;
using BenchmarkDotNet.Attributes;

namespace System.Diagnostics.Microbenchmarks;

[MemoryDiagnoser]
public class W3CBaggageDecodeBenchmarks
{
    private readonly DistributedContextPropagator _propagator = DistributedContextPropagator.CreateDefaultPropagator();
    private readonly Dictionary<string, string> _plainCarrier = new();
    private readonly Dictionary<string, string> _escapedCarrier = new();

    private static readonly DistributedContextPropagator.PropagatorGetterCallback s_getter = static (object? carrier, string fieldName, out string? value, out IEnumerable<string>? values) =>
    {
        values = null;
        ((Dictionary<string, string>)carrier!).TryGetValue(fieldName, out value);
    };

    [GlobalSetup]
    public void Setup()
    {
        // Typical unescaped baggage: short ascii key=value pairs, no '%' escapes needed.
        _plainCarrier["baggage"] = "userId=12345,sessionId=abcdef0123456789,requestType=read";

        // Baggage containing values that need percent-decoding (e.g. spaces, non-ascii-derived escapes).
        _escapedCarrier["baggage"] = "userName=John%20Doe,city=San%20Francisco,note=100%25%20done";
    }

    [Benchmark(Baseline = true)]
    public int ExtractPlainBaggage() => _propagator.ExtractBaggage(_plainCarrier, s_getter)!.Count();

    [Benchmark]
    public int ExtractEscapedBaggage() => _propagator.ExtractBaggage(_escapedCarrier, s_getter)!.Count();
}

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[Copilot]

Copilot review overview

🟡 Changes recommended

The fast path redundantly scans conforming values twice, undermining its marginal performance benefit.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Avoid double-scanning valid baggage values in fast path · New

What changed in this PR

Adds a .NET-only fast path for unescaped ASCII W3C baggage values.

Changes:

• Returns plain ASCII baggage directly without using ValueStringBuilder.
• Retains existing decoding for escaped or non-ASCII values.

File	Description
src/​libraries/​System.Diagnostics.DiagnosticSource/​src/​System/​Diagnostics/​W3CPropagator.cs	Adds the baggage-decoding fast path.

[Copilot]

Copilot review overview

🟢 Approval recommended

The optimization preserves existing behavior, uses the established character set, has existing functional coverage, and includes benchmark evidence.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Avoid double-scanning valid baggage values in fast path