perf(core): [Logs and Metrics Enable Flags 16] Avoid unused Metrics worker thread

[adinauer]

PR Stack (Logs and Metrics Enable Flags)

• feat(core): Remove Logs and Metrics enable flags #5939
• feat(logback): [Logs and Metrics Enable Flags 1] Add Logs opt-in #5940
• feat(log4j2): [Logs and Metrics Enable Flags 2] Add Logs opt-in #5941
• feat(jul): [Logs and Metrics Enable Flags 3] Add Logs opt-in #5942
• feat(android): [Logs and Metrics Enable Flags 4] Add Timber Logs opt-in #5943
• feat(android): [Logs and Metrics Enable Flags 5] Add Logcat Logs opt-in #5945
• feat(spring): [Logs and Metrics Enable Flags 6] Add logging Logs opt-in #5946
• feat(core): [Logs and Metrics Enable Flags 7] Remove Logs enable flag #5947
• feat(android): [Logs and Metrics Enable Flags 8] Warn for legacy Logs metadata #5949
• feat(core): [Logs and Metrics Enable Flags 9] Warn for legacy Logs configuration #5950
• feat(spring): [Logs and Metrics Enable Flags 10] Warn for legacy Logs property #5951
• perf(core): [Logs and Metrics Enable Flags 11] Avoid unused Logs worker thread #5952
• feat(core): [Logs and Metrics Enable Flags 12] Remove Metrics enable flag #5953
• feat(android): [Logs and Metrics Enable Flags 13] Warn for legacy Metrics metadata #5954
• feat(core): [Logs and Metrics Enable Flags 14] Warn for legacy Metrics configuration #5955
• feat(spring): [Logs and Metrics Enable Flags 15] Warn for legacy Metrics property #5956
• perf(core): [Logs and Metrics Enable Flags 16] Avoid unused Metrics worker thread #5957
• fix(jul): [Logs and Metrics Enable Flags 17] Preserve null-message records #5960
• docs: [Logs and Metrics Enable Flags 18] Update options guidance #5961
• fix(android): [Logs and Metrics Enable Flags 19] Read Timber option lazily #5964
• fix(core): [Logs and Metrics Enable Flags 20] Make migration warnings visible #5966

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📜 Description

Defers Metrics batch processor work until the processor accepts its first Metrics item.

Before first use, empty flushes and normal or restart closes do not schedule processor work. Rejected items do not mark the processor as used. After the first accepted item, batching, flush, restart-close, and Android background behavior remain unchanged.

💡 Motivation and Context

The Metrics enable flag is gone, so every initialized SDK now creates a real Metrics batch processor. Avoid starting its executor thread for applications that never capture Metrics.

💚 How did you test it?

• ./gradlew :sentry:test :sentry-android-core:testReleaseUnitTest
• ./gradlew spotlessApply apiDump
• Added lifecycle tests for construction, empty flush, close, restart close, rejected items, and first accepted use
• Added Android coverage for background callbacks before and after first use

📝 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.

#skip-changelog

⚠️ Merge this PR using a merge commit (not squash). Only the collection branch is squash-merged into main.

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.52.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	323.53 ms	373.24 ms	49.71 ms
Size	0 B	0 B	0 B

Baseline results on branch: feat/warn-legacy-metrics-spring

Startup times

Revision	Plain	With Sentry	Diff
f9a387b	320.24 ms	363.04 ms	42.80 ms

App size

Revision	Plain	With Sentry	Diff
f9a387b	0 B	0 B	0 B