Decide whether auto instrumented transactions in MAUI should set Scope.Transaction

[jamescrosswell]

Description

See:

sentry-dotnet/src/Sentry.Maui.CommunityToolkit.Mvvm/MauiCommunityToolkitMvvmEventsBinder.cs

Lines 145 to 153 in 34d8490

	// Note that we may be creating a transaction here and if so we explicitly don't store it on
	// Scope.Transaction, because Scope.Transaction is AsyncLocal<T> and MAUI Apps have a global scope. The
	// results would be that we would store the transaction on the scope, but it would never be cleared again,
	// since the next call to OnPropertyChanged for this RelayCommand will (likely) be from a different thread.
	var span = hub.StartSpan(SpanName, SpanOp);
	if (span is ITransactionTracer transaction)
	{
	hub.ConfigureScope(scope => scope.Transaction = transaction);
	}

The code comment contradicts the code implementation. Either we change/remove the comment or we change the code.

If the comment is correct then the code itself needs to be changed (seems likely but need to verify).

[linear-code]

NET-713

[Flash0ver]

We do that for ASP.NET Core already ... but .NET MAUI uses (or should at least use) a Global Scope (user code could, but should not, overwrite the default we set in our MAUI init)

Thoughts

• clicking/tapping a button that does perform a navigation
• clicking/tapping a button that does not navigate

Edge Case
.NET MAUI app on Windows, where SQL Server is used, and DiagnosticSource integration is used, which only instruments when Scope.Transaction is not null. So performance spans on MAUI would not capture DB calls.

We could also make this behavior an opt-in.

This issue becomes relevant with #5138.

Also considering https://develop.sentry.dev/sdk/foundations/state-management/scopes/

• Global Scope = Global Mode (e.g. automatically in .NET MAUI app, should be manually set in WPF app)
• Isolation = AsyncLocal flow (e.g. ASP.NET Core Requests)
• Current = with AsyncLocalScopeManager we can SentrySdk.PushScope onto the Stack when not in Global Mode

[Flash0ver]

We do want to auto-set the transaction onto the current scope, but only when there is no Transaction assigned to the scope already (scope.Transaction ??= transaction).

Considered this scenario:

• our auto-instrumentation via a Button click starts a Transaction
  • and now also sets it
• the user's event handler StartTransaction starting a new Root-Transaction (unlike StartSpan that would start a Child-Span)
  • and also uses option feat: Add SentryOptions.AutoSetScopeTransactions to auto-store transactions on the scope #5317 to automatically set this manual Transaction to the scope
• then, Scope.Transaction would still have our auto-instrumented UI-Span, rather than the user's Transaction
• users could still "force" setting "their" Transaction via ConfigureScope(scope => scope.Transaction = transaction);

[jamescrosswell]

Implemented in:

• feat: Automatic trace instrumentation for MAUI #5138
  

  sentry-dotnet/src/Sentry.Maui/Internal/MauiEventsBinder.cs

  Lines 434 to 435 in f5d5bb4

  	// Bind to scope only if no other transaction is already there (user-installed or SDK-owned navigation).
  	_hub.ConfigureScope(static (scope, tx) => scope.Transaction ??= tx, CurrentUiTx);