Logging: migrate pre-existing logs to debug logging with slf4j #1681
Description
There are some pre-existing logs in this repo. We should migrate them to the debug logging with slf4j introduced in #1586
Some extra considerations: for DefaultCredentialsProvider fine/debug level logs, perhaps consider to keep both. Also considering adding more logs for request/response that's used in the ADC flow like pingComputeEngineMetadata. context: #1586 (comment)
Note, there is a parallel ticket in sdk-platform-java for migration. These are better worked together.