fix: Introduce a new Span.Name field

[cleptric]

This removes the transaction on the scope in favor of a new Span Name field.
Besides fixing an issue with the Otel context, we also gain a cleaner way of accessing the transaction name, for example in the TracesSampler:

Before

TracesSampler: func(ctx sentry.SamplingContext) float64 {
	hub := sentry.GetHubFromContext(ctx.Span.Context())
	if hub.Scope().Transaction() == "GET /health" {
		return 0
	}
	return 1
},

After

TracesSampler: func(ctx sentry.SamplingContext) float64 {
	if ctx.Span.Name == "GET /health" {
		return 0
	}
	return 1
},

closes #596

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.12 🎉

Comparison is base (dc573d7) 78.86% compared to head (cb8009a) 78.99%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #605      +/-   ##
==========================================
+ Coverage   78.86%   78.99%   +0.12%     
==========================================
  Files          38       38              
  Lines        3870     3851      -19     
==========================================
- Hits         3052     3042      -10     
+ Misses        714      708       -6     
+ Partials      104      101       -3     

Impacted Files	Coverage Δ
scope.go	92.14% <ø> (-0.43%)	⬇️
dynamic_sampling_context.go	93.42% <100.00%> (ø)
otel/span_processor.go	94.33% <100.00%> (+2.52%)	⬆️
tracing.go	88.79% <100.00%> (+0.62%)	⬆️

... and 1 file with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[pbennett]

So how do we override the transaction name in later calls?
I do this currently:

sentry.GetHubFromContext(ctx).Scope().SetTransaction("xxxx")

This is no longer possible.

[cleptric]

transaction := sentry.TransactionFromContext(ctx)
if transaction != nil {
	transaction.Name = "xxxx"
}

[jerbob92]

How would this work for other/error events? We automatically create and inject a Hub per request and then do:

		hub.ConfigureScope(func(scope *sentry.Scope) {
			scope.SetTransaction(request_info.Id)
		})

This would then automatically add the transaction to every event coming from this scope using the ApplyToEvent method, but the lines that added it are gone:

	if scope.transaction != "" {
		event.Transaction = scope.transaction
	}

And it doesn't look an alternative has been added?

Edit: it could also be that I'm using the Transaction field completely wrong. Should I move the Request ID to something else? Like a tag or context?

[cleptric]

@jerbob92 So, transaction on the event has a long history, even before Sentry had a performance product.
There might be some legit use cases for some users that only use our error product, but in your case, a tag would give you the same UX in the product. Tags are indexed, hence searchable and appear in the UI in the same place as transaction did.

[pbennett]

Perhaps updating the documentation and SDKs are in order because frankly, transactions / spans / tags / things that show as 'database' events are all super obscure and vary from sdk to sdk.