reverted app start span timestamp to SentryLongDate

[stefanosiano]

📜 Description

reverted TimeSpan.getStartTimestamp to SentryLongDate, to not break hybrid sdks
fixed SpanFrameMetricsCollector.realNanos (it now checks the date type and reacts accordingly)
improves #3382

#skip-changelog

💡 Motivation and Context

Hybrid SDKs read the start timestamp of the app start spans as it's the current timestamp (like the SentryLongDate).
Reverting this before it gets to hybrid SDKs

💚 How did you test it?

📝 Checklist

• 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

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	375.18 ms	445.98 ms	70.80 ms
Size	1.70 MiB	2.28 MiB	592.08 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
0404ea3	394.73 ms	461.79 ms	67.06 ms
d007225	365.63 ms	438.24 ms	72.62 ms
c7e2fbc	372.00 ms	461.71 ms	89.71 ms
8838e01	387.41 ms	467.00 ms	79.59 ms
4e29063	384.14 ms	447.74 ms	63.60 ms
4e260b3	388.40 ms	468.98 ms	80.58 ms
c7e2fbc	393.98 ms	478.24 ms	84.27 ms
95a98b5	379.14 ms	420.80 ms	41.66 ms
a33b076	390.44 ms	460.29 ms	69.85 ms
2465853	422.61 ms	491.20 ms	68.58 ms

App size

Revision	Plain	With Sentry	Diff
0404ea3	1.72 MiB	2.29 MiB	577.52 KiB
d007225	1.70 MiB	2.27 MiB	586.31 KiB
c7e2fbc	1.72 MiB	2.29 MiB	576.40 KiB
8838e01	1.72 MiB	2.29 MiB	578.15 KiB
4e29063	1.72 MiB	2.29 MiB	578.38 KiB
4e260b3	1.72 MiB	2.27 MiB	554.95 KiB
c7e2fbc	1.72 MiB	2.29 MiB	576.40 KiB
95a98b5	1.70 MiB	2.27 MiB	586.31 KiB
a33b076	1.70 MiB	2.28 MiB	592.32 KiB
2465853	1.70 MiB	2.27 MiB	583.82 KiB

Previous results on branch: fix/time-span-start-date

Startup times

Revision	Plain	With Sentry	Diff
1dd46d8	418.47 ms	479.10 ms	60.64 ms

App size

Revision	Plain	With Sentry	Diff
1dd46d8	1.70 MiB	2.28 MiB	592.08 KiB

[markushi]

Tagging @buenaflor here, as this might be relevant for you.

[stefanosiano]

Tagging @buenaflor here, as this might be relevant for you.

Luckily we didn't release a new version since the previous change, so hybrid is not seeing any change