feat: Add Sampling Decision to Trace Envelope Header

[brustolin]

📜 Description

Adds sampled flag to all trace envelope headers

💡 Motivation and Context

closes #3201

💚 How did you test it?

Unit tests

📝 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

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 0205f00

[codecov]

Codecov Report

Merging #3286 (507c12a) into main (e758449) will increase coverage by 0.045%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##              main     #3286       +/-   ##
=============================================
+ Coverage   89.169%   89.215%   +0.045%     
=============================================
  Files          502       502               
  Lines        54263     54270        +7     
  Branches     19480     19488        +8     
=============================================
+ Hits         48386     48417       +31     
+ Misses        5016      4882      -134     
- Partials       861       971      +110     

Files Changed	Coverage
Sources/Sentry/SentryBaggage.m	100.000%
Sources/Sentry/SentryPropagationContext.m	100.000%
Sources/Sentry/SentryTraceContext.m	100.000%
.../SentryTests/Helper/SentrySerializationTests.swift	100.000%
...sts/SentryTests/Protocol/SentryEnvelopeTests.swift	100.000%
...s/SentryTests/Transaction/SentryBaggageTests.swift	100.000%
...entryTests/Transaction/SentryTraceStateTests.swift	100.000%

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e758449...507c12a. Read the comment docs.

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1254.94 ms	1266.38 ms	11.44 ms
Size	22.85 KiB	408.88 KiB	386.03 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
881a955	1254.14 ms	1268.43 ms	14.29 ms
1bf8571	1250.96 ms	1255.36 ms	4.40 ms
736495a	1245.16 ms	1254.42 ms	9.26 ms
d257eb9	1206.98 ms	1227.50 ms	20.52 ms
effc81c	1253.71 ms	1256.16 ms	2.45 ms
adca747	1199.37 ms	1215.49 ms	16.12 ms
a2af9fa	1236.62 ms	1253.12 ms	16.50 ms
90d17d3	1261.18 ms	1278.18 ms	17.00 ms
f8fc36d	1226.31 ms	1247.80 ms	21.49 ms
7cd187e	1196.51 ms	1226.04 ms	29.53 ms

App size

Revision	Plain	With Sentry	Diff
881a955	22.85 KiB	407.63 KiB	384.79 KiB
1bf8571	20.76 KiB	437.12 KiB	416.36 KiB
736495a	20.76 KiB	435.22 KiB	414.46 KiB
d257eb9	20.76 KiB	433.22 KiB	412.46 KiB
effc81c	20.76 KiB	433.18 KiB	412.42 KiB
adca747	20.76 KiB	401.36 KiB	380.60 KiB
a2af9fa	20.76 KiB	432.87 KiB	412.11 KiB
90d17d3	20.76 KiB	432.17 KiB	411.41 KiB
f8fc36d	20.76 KiB	419.70 KiB	398.94 KiB
7cd187e	20.76 KiB	401.66 KiB	380.90 KiB

Previous results on branch: feat/sampling-decision-trace-envelope

Startup times

Revision	Plain	With Sentry	Diff
b67b375	1242.42 ms	1249.66 ms	7.24 ms
a49138a	1250.61 ms	1261.72 ms	11.11 ms

App size

Revision	Plain	With Sentry	Diff
b67b375	22.85 KiB	407.94 KiB	385.09 KiB
a49138a	22.85 KiB	407.88 KiB	385.03 KiB

[armcknight]

Changes look fine. Though I wonder if an NSNumber encapsulating a boolean wouldn't be more appropriate than a string.

If we want to leave it as a string, we should consider using defined constants for the values.

[armcknight]

Updates look good, thanks!