Sample: update quickstart snippet

[freelerobot]

Improve the Logging quickstart in Java. Quickstart should work out of the box and contain all the instructions (in comments) so users can immediately see their logs.

Additional criteria:

• include comments on various ways to authenticate
• encourage users to log with standard log wrapper Logback rather than directly writing LogEntries. Context: In an ideal world, customers should be able to use existing logging frameworks native to their language. With some simple appender/handler to also ship logs to Cloud Logging. Customers aren't locked into logging library syntax, thus it's easier to both onboarding and offboard off of these client libraries.
• comment that if users need more customizable logs, redirect to writing log entries
• if applicable, a test for this sample

[simonz130]

@minherz this bug covers remainder samples work in the repo for structured logging in java.