fix(tracing): trust preconfigured provider for APMPlus

[feng-95]

Summary

• Treat any non-ProxyTracerProvider global tracer provider that exists before VeADK initialization as externally managing the APMPlus trace pipeline.
• Skip registering an explicitly supplied APMPlusExporter span processor in that case.
• Keep the existing ENABLE_APMPLUS=true append behavior so VeADK can still initialize its APMPlus metric uploader; the appended exporter's trace processor is not registered because tracer initialization has already completed.
• Preserve constructor-time manual APMPlus exporter registration when no global provider exists.

Why

When the OpenTelemetry plugin or application code has already installed a global tracer provider and sends traces through localhost:4318, VeADK must not add a second direct APMPlus trace pipeline. This is the minimal emergency fix and intentionally does not introduce the exporter register() mechanism.

Validation

• .venv/bin/pytest tests/test_tracing.py -q — 8 passed.
• Ruff check, Ruff format, git diff --check, pre-commit CI, gitleaks CI, and Python 3.10/3.12 unit-test CI all passed.
• Preconfigured-provider matrix covers ENABLE_APMPLUS on/off × explicit exporter present/absent, including metric-uploader retention for the env-enabled cases.
• Fresh E2E on commit 387a856 with a code-preconfigured global provider, ENABLE_APMPLUS=true, trace OTLP relay at localhost:4318, and metrics relay at localhost:4319:
  • trace 0b5f914a0297440948f592fea2f4aa64: 4 received spans, 4 unique spans, relay duplicate count 0, and APMPlus cloud query returned 4 spans;
  • exactly one trace export processor (localhost:4318) and zero VeADK direct APMPlus trace processors;
  • one APMPlus metric reader was installed; relay received the expected GenAI metrics and APMPlus cloud query returned gen_ai_chat_count=1 for the unique test service.

Known boundary

opentelemetry-instrument currently installs an empty global MeterProvider when OTEL_METRICS_EXPORTER=none. APMPlus cannot replace that provider, so this environment-specific metrics case is excluded from PR #813 acceptance; trace export remains correct and non-duplicated. This is independent of the global tracer-provider deduplication in this PR.

Follow-up

Exporter runtime registration and preserving explicitly supplied APMPlus exporters for metrics remain in the dependent Draft PR #810. After this PR merges, #810 will contain only the register increment.