Regression test: W3C traceparent upstream context in FastAPI (issue #879)

[CagriYonca]

What & Why

FastAPI >= 0.142.0 introduced its own telemetry middleware (fastapi/middleware/asyncexitstack.py
→ fastapi/telemetry/_asgi.py). When an incoming request carries a W3C traceparent header,
this middleware builds a native OpenTelemetry Context and passes it directly to
tracer.start_span(name, context=parent, ...).

Before the fix merged in PR #904, InstanaTracer.start_span() enforced a strict type check:

if parent_context and not isinstance(parent_context, SpanContext):
    raise TypeError("parent_context must be an Instana SpanContext or None.")

This caused every request carrying an upstream traceparent header to return HTTP 500,
silently breaking distributed trace continuity for any FastAPI service sitting behind another
instrumented service.

What this PR adds

A regression test — test_w3c_traceparent_upstream_context in
tests/frameworks/test_fastapi.py — that guards against this regression coming back.

The test:

1. Constructs a real W3C traceparent header with a 128-bit trace ID and 64-bit span ID.
2. Sends a GET / request via TestClient with that header.
3. Asserts HTTP 200 — a 500 here means the TypeError is still present.
4. Asserts that X-INSTANA-T, X-INSTANA-S, and Server-Timing response headers are
   set, confirming Instana's trace context is still propagated downstream.
5. Asserts that the recorded ASGI span's trace_id (lower 64 bits) matches the upstream
   trace ID, preserving cross-service trace continuity.
6. Asserts that the ASGI span's parent_id matches the upstream span ID, so the UI renders
   the correct parent–child relationship.

Test result evidence

Commit	tracer.py state	Test result
1f34ca7c (before fix)	raise TypeError(...)	❌ FAIL — HTTP 500
main (after fix, a449cebb +)	OTel → Instana SpanContext conversion	✅ PASS — HTTP 200

Scope

• No production code changed. This PR is test-only.
• The fix itself was already merged in PR fix(tracer): support converting OpenTelemetry parent span context #904 (fix(tracer): support converting OpenTelemetry parent span context).

Related

• Issue: [Bug]: InstanaTracer.start_span() missing context keyword argument — incompatible with OpenTelemetry Tracer API #879
• Fix PR: fix(tracer): support converting OpenTelemetry parent span context #904