diagnostics_channel: lazily create tracing context

[RomainLanz]

Note

AI assistance was used to investigate this issue and draft parts of the
implementation, tests, benchmarks, and PR description. I reviewed and
verified the final changes and benchmark results.

Hey! 👋🏻

While benchmarking our HTTP router, I noticed that manually checking whether a diagnostics channel had subscribers before tracing an operation improved performance.

That felt unexpected, so I dug deeper. It turns out that TracingChannel.traceSync() and tracePromise() create their default context before checking whether any tracing channel has subscribers.

As a result, calls without an explicit context allocate an object even when they immediately take the no-subscriber fast path.

This PR moves the default-context creation after the subscriber check. It also adds focused tests and benchmarks to verify the behavior and measure the performance impact.

Benchmarks

The comparison used two release builds, 30 alternating runs per configuration,
n=3,000,000, and benchmark/compare.js --analyze.

Method	Context	Improvement	Confidence interval
traceSync	omitted	+59.94%	±2.11%
traceSync	undefined	+59.48%	±3.54%
tracePromise	omitted	+42.03%	±4.85%
tracePromise	undefined	+43.62%	±3.38%

All four improvements have *** confidence (p < 0.001). No statistically
significant regressions were observed for explicit-context or subscribed
control cases.

The comparison can be reproduced with:

node benchmark/compare.js \
  --old ./node-before \
  --new ./node-after \
  --runs 30 \
  --filter tracing-channel- \
  --analyze \
  diagnostics_channel

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.16%. Comparing base (18fb35a) to head (b86b81a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65513      +/-   ##
==========================================
+ Coverage   90.15%   90.16%   +0.01%     
==========================================
  Files         751      751              
  Lines      253514   253557      +43     
  Branches    47746    47774      +28     
==========================================
+ Hits       228563   228630      +67     
+ Misses      16203    16195       -8     
+ Partials     8748     8732      -16     

Files with missing lines	Coverage Δ
lib/diagnostics_channel.js	97.34% <100.00%> (+0.03%)	⬆️

... and 32 files with indirect coverage changes

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