Speed up local parametric testing capabilities by 2-5x

[bm1549]

Motivation

Re-running parametric tests (e.g. when iterating on a test or debugging) was slow because each run could rebuild the parametric library image and, with multiple workers, start a new test agent and library container per test. This PR documents and implements options so parametric re-runs are faster: single-worker mode (+1) for shared agent and library container reuse, and --skip-parametric-build to skip rebuilding the image when only test code or docs change.

Runtime impact (parametric re-runs)

Measured on a single parametric test: tests/parametric/test_tracer.py::Test_Tracer::test_tracer_span_top_level_attributes) with TEST_LIBRARY=nodejs

On a secondary run, using +1 and --skip-parametric-build is about 3-7× faster in pytest time and ~2-5× faster in wall time, from skipping the parametric image build and reusing one agent and one library container.

	Before	After
Pytest time	~7.3s	~1.1-4.5s
Wall time	~10s	~2-6s

Changes

• Docs: Added “Making parametric runs faster” in docs/scenarios/parametric.md; added .cursor/rules/parametric-testing.mdc and updated AGENTS.md to recommend +1 and --skip-parametric-build for re-runs.
• run.sh: Documented +1 and that single-worker PARAMETRIC runs reuse one agent and one library container per session.
• Framework: Added --skip-parametric-build (and SKIP_PARAMETRIC_BUILD) in conftest.py and utils/_context/_scenarios/parametric.py; in utils/docker_fixtures/ implemented session-scoped agent and library container reuse for parametric when using a single worker.

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

.cursor/rules/parametric-testing.mdc                                    @DataDog/system-tests-core
AGENTS.md                                                               @DataDog/system-tests-core
conftest.py                                                             @DataDog/system-tests-core
docs/scenarios/parametric.md                                            @DataDog/system-tests-core
run.sh                                                                  @DataDog/system-tests-core
tests/parametric/conftest.py                                            @DataDog/system-tests-core @DataDog/apm-sdk-capabilities
utils/_context/_scenarios/parametric.py                                 @DataDog/system-tests-core
utils/docker_fixtures/_core.py                                          @DataDog/system-tests-core
utils/docker_fixtures/_test_agent.py                                    @DataDog/system-tests-core
utils/docker_fixtures/_test_clients/_test_client_parametric.py          @DataDog/system-tests-core

[nccatoni]

These changes could cause reliability issues. While it might be fine to speedup local development you should make sure that all parametric tests do indeed pass. You should also get a review from @cbeauchesne (OOO this week)

[nccatoni]

Are you sure that docker doesn't already cache the images in this case ?

[bm1549]

Going based on empirical evidence, it saved an additional ~1-2 seconds per run since we avoid the layer cache validation