Fix applying the trace context on events

[cleptric]

Fixes #884

We mistakenly used the scope's span trace context as the trace context for the transaction. Instead, we now set the trace context on the event when the transaction finishes and only use the scope's span trace context in case there is no trace context on the event already.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.82%. Comparing base (951e363) to head (60da897).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scope.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
- Coverage   82.85%   82.82%   -0.03%     
==========================================
  Files          55       55              
  Lines        4619     4623       +4     
==========================================
+ Hits         3827     3829       +2     
- Misses        636      637       +1     
- Partials      156      157       +1     

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[Darevski]

@cleptric This fix is a partially functional solution that fixes the trace context only for transactions.

It does not resolve the tracing information for events such as messages, exceptions, or any other attributes related to scope behavior (like breadcrumbs, etc.) based on the starting span setting here https://github.com/getsentry/sentry-go/blob/master/tracing.go#L197-L198.

Here is an example of how the following code behaves:

        // Start the parent transaction
	ctx := context.Background()
	parentTransaction := sentry.StartTransaction(ctx, "parent-transaction")

	// Start the parent transaction span
	parentSpan := sentry.StartSpan(parentTransaction.Context(), "parent-span")

	parentChildSpan := sentry.StartSpan(parentSpan.Context(), "parent-child-span")

	// Capture message that should be attached to "parent-child-span"
	sentry.CaptureMessage("Parent child span")

	// Finish the  "parent-child-span"
	parentChildSpan.Finish()

	// Capture message that should be attached to "parent-span", but will be attached to the "parent-child-span"
	sentry.CaptureMessage("Parent span")

	// Finish the "parent span"
	parentSpan.Finish()

	// Capture message that should be attached to "parent-transaction", but will be attached to the "parent-child-span"
	sentry.CaptureMessage("Parent transaction completed")

	parentTransaction.Finish()

On the right side is the code running with PR #886, and on the left side is the code running with the current PR #888.
In #888 all sentry events are related to the parent-child-span, but its wrong.

2024-10-08.09.41.04.mov

[szokeasaurusrex]

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