perf: Collect performance data on shared timer executor (JAVA-653)

[runningcode]

📜 Description

DefaultCompositePerformanceCollector created a java.util.Timer (a dedicated thread) whenever performance collection started, and cancelled it when the last transaction finished — so every burst of transactions created and destroyed a thread. The periodic 100ms collection now runs on the shared timer executor (SentryOptions#getTimerExecutorService):

• Each collection run reschedules itself after TRANSACTION_COLLECTION_INTERVAL_MILLIS, reproducing Timer's fixed-delay behavior.
• A generation counter (guarded by the existing lock) makes sure an in-flight run doesn't reschedule itself after close() — including when the run itself stops a timed-out transaction and thereby closes the collector.
• close() cancels the pending tick via Future#cancel(false), matching the previous Timer#cancel() semantics.

💡 Motivation and Context

Part of reducing the number of threads created by the SDK: JAVA-653.

Apps with tracing enabled paid for a thread creation/destruction cycle per transaction burst, plus a live extra thread while transactions ran. The shared timer executor's single worker is reused across all timeouts and self-terminates when idle.

💚 How did you test it?

Existing unit tests, adapted from Timer-mock verifications to the executor/future model (DefaultCompositePerformanceCollectorTest), plus SentryTracerTest and AndroidOptionsInitializerTest.

📝 Checklist

• I added GH Issue ID & Linear ID
• 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.
• I updated the wizard 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

Related PRs in this effort: RateLimiter (#5814), LifecycleWatcher (#5819), HostnameCache (#5817), batch processors (#5818).

🤖 Generated with Claude Code

[linear-code]

JAVA-653

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.50.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	357.18 ms	490.18 ms	133.00 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
eb95ded	317.51 ms	369.08 ms	51.57 ms
2124a46	319.19 ms	415.04 ms	95.85 ms
c3ee041	310.64 ms	361.90 ms	51.26 ms
dcc6bbf	382.58 ms	462.13 ms	79.54 ms
7a19fee	315.46 ms	368.62 ms	53.16 ms
9054d65	330.94 ms	403.24 ms	72.30 ms
d500866	326.13 ms	378.70 ms	52.58 ms
ad8da22	365.86 ms	427.00 ms	61.14 ms
ed33deb	337.52 ms	484.06 ms	146.54 ms
d15471f	310.26 ms	377.04 ms	66.78 ms

App size

Revision	Plain	With Sentry	Diff
eb95ded	0 B	0 B	0 B
2124a46	1.58 MiB	2.12 MiB	551.51 KiB
c3ee041	0 B	0 B	0 B
dcc6bbf	1.58 MiB	2.12 MiB	553.10 KiB
7a19fee	0 B	0 B	0 B
9054d65	1.58 MiB	2.29 MiB	723.38 KiB
d500866	0 B	0 B	0 B
ad8da22	1.58 MiB	2.29 MiB	719.83 KiB
ed33deb	1.58 MiB	2.13 MiB	559.52 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB