[WIP] OTLP trace metrics export

[ida613]

OTLP trace metrics export:
tests/otel/test_otlp_trace_metrics.py validates that all four
dd.trace.span.* metrics appear on /v1/metrics when
DD_TRACE_OTEL_METRICS_ENABLED=true (new APM_TRACING_OTLP_METRICS scenario).

W3C TraceState ot.th injection:
tests/parametric/test_tracestate_ot_th.py validates that an ot=th:
vendor entry is present in the outgoing tracestate for sampled spans.

Supporting changes:

• Add apm_tracing_otlp_metrics scenario (scenarios/init.py)
• Add get_otel_metrics() to OpenTelemetryInterfaceValidator (interfaces)
• Add otlp_trace_metrics and w3c_tracestate_ot_probability_threshold features
• Mark both tests as missing_feature in manifests/nodejs.yml

Motivation

Changes

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:

tests/otel/test_otlp_trace_metrics.py                                   @DataDog/system-tests-core
tests/parametric/test_tracestate_ot_th.py                               @DataDog/system-tests-core @DataDog/apm-sdk-capabilities
manifests/nodejs.yml                                                    @DataDog/dd-trace-js
utils/_context/_scenarios/__init__.py                                   @DataDog/system-tests-core
utils/_features.py                                                      @DataDog/system-tests-core
utils/interfaces/_open_telemetry.py                                     @DataDog/system-tests-core