fix: App start ends when first frame is drawn

[philipphofmann]

When performanceV2 is enabled, the app start ends when the app draws the first frame instead of when it receives the
UIWindowDidBecomeVisibleNotification.

This PR is based on #3525.

📜 Description

💡 Motivation and Context

This the change requested in #3461, but in a non-breaking way behind the feature flag performanceV2.

💚 How did you test it?

Unit tests and simulator.

📝 Checklist

You have to check all boxes before merging:

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1a4da1b) 89.159% compared to head (794fe78) 89.258%.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3530       +/-   ##
=============================================
+ Coverage   89.159%   89.258%   +0.098%     
=============================================
  Files          528       528               
  Lines        57527     57600       +73     
  Branches     20609     20630       +21     
=============================================
+ Hits         51291     51413      +122     
- Misses        5200      5276       +76     
+ Partials      1036       911      -125     

Files	Coverage Δ
Sources/Sentry/SentryAppStartTracker.m	97.590% <100.000%> (+1.461%)	⬆️
Sources/Sentry/SentryAppStartTrackingIntegration.m	100.000% <100.000%> (ø)
Sources/Sentry/SentryOptions.m	97.321% <100.000%> (+0.018%)	⬆️
Sources/Sentry/SentryTracer.m	97.159% <100.000%> (ø)
.../AppStartTracking/SentryAppStartTrackerTests.swift	98.481% <100.000%> (+1.205%)	⬆️
...cking/SentryAppStartTrackingIntegrationTests.swift	92.523% <100.000%> (+0.856%)	⬆️
Tests/SentryTests/SentryOptionsTest.m	97.811% <100.000%> (+0.007%)	⬆️
...ts/SentryTests/Transaction/SentryTracerTests.swift	98.874% <100.000%> (+0.015%)	⬆️

... and 39 files with indirect coverage changes

---

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1225.35 ms	1241.34 ms	15.99 ms
Size	21.58 KiB	418.60 KiB	397.02 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
ab0012c	1209.06 ms	1228.78 ms	19.72 ms
51307b7	1223.08 ms	1240.76 ms	17.68 ms
15cf6bf	1241.06 ms	1248.18 ms	7.12 ms
3437454	1254.04 ms	1259.50 ms	5.46 ms
25bcc50	1209.53 ms	1224.86 ms	15.33 ms
984eb2d	1220.62 ms	1235.24 ms	14.62 ms
d80d410	1231.50 ms	1240.14 ms	8.64 ms
7192d9e	1221.86 ms	1248.28 ms	26.42 ms
7bb0873	1215.65 ms	1235.00 ms	19.35 ms
e7b566f	1199.57 ms	1218.76 ms	19.19 ms

App size

Revision	Plain	With Sentry	Diff
ab0012c	22.85 KiB	415.09 KiB	392.24 KiB
51307b7	22.85 KiB	407.63 KiB	384.78 KiB
15cf6bf	22.85 KiB	405.38 KiB	382.53 KiB
3437454	22.85 KiB	408.87 KiB	386.02 KiB
25bcc50	20.76 KiB	427.22 KiB	406.46 KiB
984eb2d	20.76 KiB	425.77 KiB	405.01 KiB
d80d410	20.76 KiB	425.71 KiB	404.95 KiB
7192d9e	20.76 KiB	431.71 KiB	410.95 KiB
7bb0873	22.85 KiB	407.09 KiB	384.24 KiB
e7b566f	22.85 KiB	414.80 KiB	391.95 KiB

Previous results on branch: fix/app-start-duration

Startup times

Revision	Plain	With Sentry	Diff
6b310a3	1197.22 ms	1211.82 ms	14.60 ms
2cfa65e	1198.80 ms	1213.63 ms	14.84 ms

App size

Revision	Plain	With Sentry	Diff
6b310a3	21.58 KiB	418.67 KiB	397.08 KiB
2cfa65e	21.58 KiB	418.67 KiB	397.08 KiB

[brustolin]

LGTM

Just wondering what's the relevance of this now that it aligns with TTID, both info will be always the same.

[philipphofmann]

Just wondering what's the relevance of this now that it aligns with TTID, both info will be always the same.

Yeah, I had the same thought. I don't think this is necessarily bad, but it's redundant. Maybe we need a different concept for app starts. I will keep this in the back of my head and bring it up in our Mobile Starfish discussions.

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

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## Unreleased

- App start ends when first frame is drawn ([#3530](https://github.com/getsentry/sentry-cocoa/pull/3530))

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