Store sampling context in the AsyncContextFrame directly

[szegedi]

What does this PR do?:

• Discards the CPED approach where we replace the AsyncContextFrame (ACF) object in the CPED with a proxy that has an internal field for our sample context.
• Instead, adopts an approach where the sample context is stored directly in the ACF map under an arbitrary key.
• It allows passing in a key externally. TypeScript layer passes in an AsyncLocalStorage as the key so the ACF map remains homogenous in its keys.

Motivation:
Simplification and performance.
Simplification: Storing the sampling context directly in the ACF reduces the runtime complexity by not having a proxy object. Arguably, there's the added complexity of having the raw memory map reading code, but there's only so much complexity you can reduce and the remaining inherent complexity you can only push around from one place to another.
Performance: We also no longer need to ensure that the proxy object is updated every time a new async context is created, thus dd-trace-js no longer needs to instrument AsyncLocalStorage.enterWith, eliminating the need to do any JS work in any callback on context creation or switches. I think this is pretty significant. See https://github.com/DataDog/dd-trace-js/tree/szegedi/acf-simple for the dd-trace-js side of changes.

Additional Notes:
While storing the sampling context is now simpler, retrieving it from the signal handler actually got more complex, but that's a tradeoff we should make. Namely, in the signal handler now we need to chase pointers from current isolate to the hash table backing the ACF storage, and then also implement the hash table lookup-by-key. None of these data structures are published by V8, but we're defining equivalent ones that hold for the currently used V8 versions. They would also need to be updated if V8 internally changes them, but we have enough tests to detect if they no longer work.

How to test the change?
All existing CPED tests pass, and I've added some additional tests in test-get-value-from-map-profiler.ts.

[github-actions]

Overall package size

Self size: 1.87 MB
Deduped: 2.24 MB
No deduping: 2.24 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | source-map | 0.7.6 | 185.63 kB | 185.63 kB | | pprof-format | 2.2.1 | 163.06 kB | 163.06 kB | | p-limit | 3.1.0 | 7.75 kB | 13.78 kB | | node-gyp-build | 3.9.0 | 8.81 kB | 8.81 kB |

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-02-20 10:46:21

Comparing candidate commit 6dd0ed8 in PR branch szegedi/acf-simple with baseline commit e88303f in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 91 metrics, 29 unstable metrics.

[nsavoire]

@codex review

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[nsavoire]

@codex review

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Reviewed commit: 889d79f2aa

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[nsavoire]

Impressive work !