TTID is wrong when viewWillAppear isn't called

[philipphofmann]

Description

When the OS doesn't call viewWillAppear the TTID span gets finished when it's tracer finishes:

sentry-cocoa/Sources/Sentry/SentryTimeToDisplayTracker.m

Lines 97 to 101 in 8c38fb1

	// The tracer finishes when the screen is fully displayed. Therefore, we must also finish
	// the TTID span.
	if (self.initialDisplaySpan.isFinished == NO) {
	[self.initialDisplaySpan finish];
	}

This is because the reportInitialDisplay only gets called from the SentryUIViewControllerPerformanceTracker when viewWillAppear gets called:

sentry-cocoa/Sources/Sentry/SentryUIViewControllerPerformanceTracker.m

Line 191 in 8c38fb1

[ttdTracker reportInitialDisplay];

There are scenarios for which the OS doesn't call viewWillAppear; see Apple docs. We could call reportInitialDisplay also in subsequent UIViewController methods that the OS calls before rendering the first frame of the UIViewController, such as viewIsAppearing or viewWillLayoutSubView, etc.

Especially when having performanceV2 enabled, this leads to confusion because the initial frame render of the app start differs from the TTID. The app start metric subscribes to the very first frame being rendered

sentry-cocoa/Sources/Sentry/SentryAppStartTracker.m

Lines 68 to 71 in 8c38fb1

	if (_enablePerformanceV2) {
	self.framesTracker = framesTracker;
	[framesTracker addListener:self];
	}

Maybe we need to align the initial frame render of the app start time with the TTID when fixing this bug.