fix: propagators/aws/xray: handle ALB headers missing Parent field (#8024)

[piy3]

AWS ALB injects X-Amzn-Trace-Id with only a Root field when it is the first node in the request chain — no Parent span exists yet.For Example :
X-Amzn-Trace-Id: Root=1-68ef9936-5f4df7cb590e654d2f659b2c

Before
The Extract() method guarded context propagation with sc.IsValid(), which is defined in the OTel SDK as:
func (sc SpanContext) IsValid() bool { return sc.HasTraceID() && sc.HasSpanID() // both must be non-zero}
Since there is no Parent field in the ALB header, SpanID stays all-zeros after parsing. sc.IsValid() returned false, so the trace context was silently dropped and the downstream service started a completely new unrelated trace — breaking trace continuity.
After
Changed the guard to sc.TraceID().IsValid(), which only requires the TraceID to be non-zero:
if err == nil && sc.TraceID().IsValid() { return trace.ContextWithRemoteSpanContext(ctx, sc)}
Now a header carrying only Root is accepted and the TraceID is correctly propagated into the context. The downstream service creates a root span that belongs to the existing ALB-originated trace instead of starting a new one.
Tests
Added three test cases covering:
Root only (no Parent, no Sampled)
Root + Sampled=1 (no Parent)
Root + Sampled=0 (no Parent)

[linux-foundation-easycla]

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• ✅ login: piy3 / name: Piyush Yadav (d413eef)

[pellared]

AWS ALB injects X-Amzn-Trace-Id with only a Root field when it is the first node in the request chain — no Parent span exists yet.

This seems an improper behavior of AWS ALB. How would you have a trace without a span?

CC @akats7

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (5995ae8) to head (d413eef).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8615   +/-   ##
=====================================
  Coverage   82.2%   82.2%           
=====================================
  Files        179     179           
  Lines      13724   13724           
=====================================
  Hits       11289   11289           
  Misses      2033    2033           
  Partials     402     402           

Files with missing lines	Coverage Δ
propagators/aws/xray/propagator.go	97.3% <100.0%> (ø)

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