[Java 9.x] Replace global Logs/Metrics flags with opt-ins #6213

For the 9.x major, remove the global enableLogs / enableMetrics options and replace automatic log collection gating with integration-local opt-ins (Logback, Log4j2, JUL, Spring Boot, Android Timber/Logcat), disabled by default.

Open PR stack, targeting 9.x.x (collection PR #5939, squash-merged once the stack lands):

1. feat(logback): [Logs and Metrics Enable Flags 1] Add Logs opt-in #5940 Logback Logs opt-in
2. feat(log4j2): [Logs and Metrics Enable Flags 2] Add Logs opt-in #5941 Log4j2 Logs opt-in
3. feat(jul): [Logs and Metrics Enable Flags 3] Add Logs opt-in #5942 JUL Logs opt-in
4. feat(android): [Logs and Metrics Enable Flags 4] Add Timber Logs opt-in #5943 Android Timber Logs opt-in
5. feat(android): [Logs and Metrics Enable Flags 5] Add Logcat Logs opt-in #5945 Android Logcat Logs opt-in
6. feat(spring): [Logs and Metrics Enable Flags 6] Add logging Logs opt-in for Spring Boot #5946 Spring Boot logging Logs opt-in
7. feat(core): [Logs and Metrics Enable Flags 7] Remove Logs enable flag #5947 Remove Logs enable flag
8. perf(core): [Logs and Metrics Enable Flags 8] Avoid unused Logs worker thread #5952 Avoid unused Logs worker thread
9. feat(core): [Logs and Metrics Enable Flags 9] Remove Metrics enable flag #5953 Remove Metrics enable flag
10. perf(core): [Logs and Metrics Enable Flags 10] Avoid unused Metrics worker thread #5957 Avoid unused Metrics worker thread
11. fix(jul): [Logs and Metrics Enable Flags 11] Preserve null-message records #5960 JUL: preserve null-message records
12. docs: [Logs and Metrics Enable Flags 12] Update options guidance #5961 Update options guidance
13. fix(android): [Logs and Metrics Enable Flags 13] Read Timber option lazily #5964 Android: read Timber option lazily
14. ref(logging): [Logs and Metrics Enable Flags 14] Rename Logs opt-ins #5970 Rename Logs opt-ins

Related: #5364 / #5364, Android #5363.

Action taken on behalf of Alexander Dinauer.