trace/auto: pass span context to auto-instrumentation start hook

[RonFed]

The auto-instrumentation start hook can use the context hierarchy to track relationships between spans, the way the SDK does with SpanFromContext.
autoTracer.Start calls the hook before ContextWithSpan, so the hook receives the parent's context rather than the new span's. When sibling spans started from the same context overlap in time, they then look like one span is the parent of the other.

ctx, parent := tracer.Start(ctx, "parent")
_, a := tracer.Start(ctx, "a")
_, b := tracer.Start(ctx, "b") // expected child of "parent", reported as child of "a"
b.End()
a.End()
parent.End()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.1%. Comparing base (9e72307) to head (40ce169).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #9029   +/-   ##
=====================================
  Coverage   89.1%   89.1%           
=====================================
  Files        338     338           
  Lines      22274   22274           
=====================================
+ Hits       19854   19856    +2     
+ Misses      2420    2418    -2     

Files with missing lines	Coverage Δ
trace/auto.go	98.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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[ps-mir]

Change looks ok, suggestion for additional test and changelog entry.

[RonFed]

thanks @ps-mir, addressed your comments