migrate existing logged warnings to slf4j logs #3647
Description
Debug logging are introduced in #3403. We have existing undocumented log entries (mostly for warnings) that needs to be migrated.
- Refactor exposed logging classes, to improve readability and make it easier to maintain. context: https://github.com/googleapis/sdk-platform-java/pull/3403/files#r1964339761
- Expose a generic logging method that can be used for more general purpose logging, so that we can migrate existing warnings to it.
- Analyse and migrate existing logs (this should cover both auth and gax. determine if google-cloud-java is also in scope for migration)