http2: speed up response getHeaders

[rozzilla]

Replace ObjectAssign in Http2ServerResponse.getHeaders() with an explicit
key-copy loop, preserving the null-prototype shallow copy. Add a benchmark
and compatibility coverage.

On Apple M2 / macOS, compared against
4fa5145:

Headers	Throughput improvement
0	+37.30%
1	+149.27%
4	+180.65%
16	+229.59%
64	+256.02%

30 samples per binary and configuration, with 1,000,000 calls per sample.
These results measure getHeaders() throughput, not HTTP request throughput.

[nodejs-github-bot]

Review requested:

• @nodejs/http2
• @nodejs/net
• @nodejs/performance

[github-actions]

Welcome to Node.js, and thank you for your first contribution!

Before review, please take a moment to read:

• the guide for first-time contributors
• the contribution and automation policies
• the pull request guide
• the AI use policy
• the Code of Conduct

Please make sure every commit is signed off. For a first pull request, GitHub Actions require collaborator approval and Jenkins CI must be started by a collaborator or triager, so an initial wait is normal.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.45%. Comparing base (554dd55) to head (3362b41).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66592      +/-   ##
==========================================
+ Coverage   90.44%   90.45%   +0.01%     
==========================================
  Files         791      791              
  Lines      276486   276568      +82     
  Branches    53092    53117      +25     
==========================================
+ Hits       250060   250169     +109     
+ Misses      16846    16802      -44     
- Partials     9580     9597      +17     

Files with missing lines	Coverage Δ
lib/internal/http2/compat.js	97.25% <100.00%> (+0.01%)	⬆️

... and 38 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[pimterry]

LGTM

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/78262/

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/78273/

[github-actions]

Benchmark GHA (http2 / get-headers.js): https://github.com/nodejs/node/actions/runs/38037263644

Results

[!WARNING]
Do not take GHA benchmark results as face value, always confirm them
using a dedicated machine, e.g. Jenkins CI.

Benchmark results:

                                          confidence improvement accuracy (*)    (**)   (***)
http2/get-headers.js headers=0 n=1000000         ***     33.00 %      ±12.38% ±16.33% ±20.95%
http2/get-headers.js headers=1 n=1000000         ***    186.95 %      ±26.41% ±34.89% ±44.90%
http2/get-headers.js headers=16 n=1000000        ***    262.96 %      ±30.59% ±40.41% ±52.04%
http2/get-headers.js headers=4 n=1000000         ***    224.45 %      ±28.45% ±37.59% ±48.39%
http2/get-headers.js headers=64 n=1000000        ***    298.00 %      ±32.70% ±43.21% ±55.65%

Be aware that when doing many comparisons the risk of a false-positive
result increases. In this case, there are 5 comparisons, you can thus
expect the following amount of false-positive results:
  0.25 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.05 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)

[!WARNING]
Do not take GHA benchmark results as face value, always confirm them
using a dedicated machine, e.g. Jenkins CI.

[nodejs-github-bot]

Landed in b823c3a