perf: gather profiler metrics gathering on a low-pri queue

[armcknight]

📜 Description

Change the metric profiler from using an NSTimer, which dispatches blocks to the main thread, to a dispatch source that dispatches blocks to a queue with a utility quality of service.

💡 Motivation and Context

We noticed some customer profiles that showed our own symbols doing lots of work on main threads. This is undesirable from a customer POV and isn't even really necessary for these readings.

💚 How did you test it?

Created mocking for the dispatch source and verified the preexisting tests still work.

This also introduces the aforementioned dispatch factory that vends our wrappers for dispatch queues and the new source wrapper, which we should eventually use in all the places in the SDK that internally alloc/init queue wrappers, for easier and more consistent mockability in 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

[armcknight]

This is no longer true since we bumped our min supported version. I believe we can also change this:

sentry-cocoa/Sources/Sentry/SentryDispatchQueueWrapper.m

Lines 16 to 17 in d257eb9

	// DISPATCH_QUEUE_SERIAL_WITH_AUTORELEASE_POOL is requires iOS 10. Since we are targeting
	// iOS 9 we need to manually add the autoreleasepool.

This is now exposed as a property in the public interface so it can be accessed when setting up a new dispatch_source_t in SentryDispatchSourceWrapper[

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1227.36 ms	1249.04 ms	21.68 ms
Size	20.76 KiB	436.51 KiB	415.75 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
3e7aa41	1214.02 ms	1238.84 ms	24.82 ms
d253cdf	1231.61 ms	1259.52 ms	27.91 ms
d60f70a	1219.63 ms	1228.54 ms	8.91 ms
cf724da	1243.14 ms	1261.44 ms	18.30 ms
d1d5c2d	1239.20 ms	1251.92 ms	12.72 ms
cb2eefe	1222.04 ms	1243.04 ms	21.00 ms
ecd9ecd	1204.67 ms	1226.46 ms	21.79 ms
2cf1b84	6265.25 ms	6277.98 ms	12.73 ms
25bcc50	1240.47 ms	1254.70 ms	14.23 ms
10ee2ce	1250.90 ms	1258.57 ms	7.67 ms

App size

Revision	Plain	With Sentry	Diff
3e7aa41	20.76 KiB	432.31 KiB	411.55 KiB
d253cdf	20.76 KiB	427.66 KiB	406.90 KiB
d60f70a	20.76 KiB	430.97 KiB	410.21 KiB
cf724da	20.76 KiB	430.52 KiB	409.76 KiB
d1d5c2d	20.76 KiB	427.83 KiB	407.07 KiB
cb2eefe	20.76 KiB	419.62 KiB	398.86 KiB
ecd9ecd	20.76 KiB	420.23 KiB	399.47 KiB
2cf1b84	20.76 KiB	431.91 KiB	411.15 KiB
25bcc50	20.76 KiB	427.22 KiB	406.46 KiB
10ee2ce	20.76 KiB	427.77 KiB	407.00 KiB

Previous results on branch: armcknight/perf/metric-profiler-nonmain-thread

Startup times

Revision	Plain	With Sentry	Diff
fcf88c3	1244.68 ms	1253.80 ms	9.12 ms
9584752	1218.84 ms	1233.46 ms	14.62 ms
3c724c4	1246.64 ms	1250.74 ms	4.10 ms

App size

Revision	Plain	With Sentry	Diff
fcf88c3	20.76 KiB	433.87 KiB	413.11 KiB
9584752	20.76 KiB	435.54 KiB	414.78 KiB
3c724c4	20.76 KiB	435.56 KiB	414.80 KiB

[indragiek]

recordCPUPercentagePerCore and recordMemoryFootprint were previously called on the main thread, are they thread safe?

[armcknight]

Good question. Their calls to task_info and host_processor_info are not synchronized, but their modifications of the data structure those readings are stored in are inside @synchronized blocks. Is this safe, or do you think we should move the task_info/host_processor_info calls inside the synchronization as well?

[armcknight]

i improved the method that reads from the data structure as well in adef4bc

[philipphofmann]

Sorry, @armcknight, I didn't get to this today. I will try to have a look tomorrow. In the meantime maybe you have time to fix CI 😃?

[brustolin]

LGTM.

A few suggestions to think about.

[philipphofmann]

Not reviewing this, as @brustolin already LGTM.

[github-actions]

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