fix: TTID/TTFD for app start transactions

[philipphofmann]

📜 Description

The TTFD/TTID spans and measurements for app start transactions were wrong, as they didn't include the app start duration. This is fixed now.

This PR is based on #3505.

💡 Motivation and Context

Fixes GH-3471

💚 How did you test it?

📝 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

Merging #3512 (2b6f588) into main (fac579e) will increase coverage by 0.017%.
The diff coverage is 100.000%.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3512       +/-   ##
=============================================
+ Coverage   89.205%   89.223%   +0.017%     
=============================================
  Files          528       528               
  Lines        57476     57570       +94     
  Branches     20608     20630       +22     
=============================================
+ Hits         51272     51366       +94     
  Misses        5294      5294               
  Partials       910       910               

Files	Coverage Δ
Sources/Sentry/SentryTimeToDisplayTracker.m	100.000% <100.000%> (ø)
Sources/Sentry/SentryTracer.m	96.353% <100.000%> (ø)
...iewController/SentryTimeToDisplayTrackerTest.swift	99.696% <100.000%> (+0.077%)	⬆️

... and 17 files with indirect coverage changes

---

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1209.96 ms	1234.44 ms	24.48 ms
Size	21.58 KiB	418.44 KiB	396.86 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
dc0db9e	1222.10 ms	1240.90 ms	18.80 ms
455619d	1253.08 ms	1265.06 ms	11.98 ms
279841c	1250.80 ms	1263.08 ms	12.29 ms
c78683b	1246.71 ms	1258.70 ms	11.99 ms
156e771	1228.06 ms	1242.64 ms	14.58 ms
67460f4	1244.56 ms	1255.96 ms	11.40 ms
6bc5ae5	1230.90 ms	1246.10 ms	15.20 ms
b9b0f0a	1239.76 ms	1257.68 ms	17.92 ms
8f397a7	1236.76 ms	1256.76 ms	20.00 ms
4afae53	1217.65 ms	1229.27 ms	11.62 ms

App size

Revision	Plain	With Sentry	Diff
dc0db9e	20.76 KiB	419.62 KiB	398.86 KiB
455619d	20.76 KiB	432.87 KiB	412.11 KiB
279841c	22.84 KiB	403.19 KiB	380.34 KiB
c78683b	20.76 KiB	435.24 KiB	414.48 KiB
156e771	20.76 KiB	419.70 KiB	398.94 KiB
67460f4	20.76 KiB	426.15 KiB	405.39 KiB
6bc5ae5	20.76 KiB	401.40 KiB	380.64 KiB
b9b0f0a	20.76 KiB	434.94 KiB	414.18 KiB
8f397a7	20.76 KiB	420.55 KiB	399.79 KiB
4afae53	22.84 KiB	402.08 KiB	379.24 KiB

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

Startup times

Revision	Plain	With Sentry	Diff
5e04568	1227.04 ms	1254.51 ms	27.47 ms

App size

Revision	Plain	With Sentry	Diff
5e04568	21.58 KiB	418.38 KiB	396.80 KiB

[brustolin]

LGTM

[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.

Example:

## Unreleased

- TTID/TTFD for app start transactions ([#3512](https://github.com/getsentry/sentry-cocoa/pull/3512))

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