feat(core): [Logs and Metrics Enable Flags 14] Warn for legacy Metrics configuration - #5955

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

📜 Description

Continues parsing nullable metrics.enabled external configuration and emits tailored migration warnings during SentryOptions.merge for explicit true and false values.

The obsolete value is not applied. Manual Sentry.metrics() capture remains active for either value, while absent configuration emits no warning.

💡 Motivation and Context

The aggregate Metrics enable flag has been removed. Keeping nullable external parsing temporarily lets users of sentry.properties, environment variables, and system properties receive a targeted migration warning instead of having obsolete configuration silently ignored.

💚 How did you test it?

• ./gradlew :sentry:test
• ./gradlew spotlessApply apiDump
• Added tests for absent, explicit true, and explicit false configuration
• Verified both legacy values leave manual Metrics capture active

📝 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

Add equivalent migration warnings for the legacy Spring Boot Metrics property.

#skip-changelog

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