chore: comment about how to instantiate logger variable in Cloud Run

[suztomo]

This App.java is referenced in https://cloud.google.com/run/docs/samples/cloudrun-manual-logging#cloudrun_manual_logging-java. But the document does not explain how to instantiate and configure the logger variable.

https://cloud.google.com/run/docs/logging#run_manual_logging-java has the answer. So let's add the explanation in the source code comment.

Description

Fixes #

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Checklist

• [ ] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] Tests pass: mvn clean verify required
• [ ] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved

[suztomo]

@iennae Thank you for quick response. Now I see https://cloud.google.com/run/docs/samples/cloudrun-manual-logging#cloudrun_manual_logging-java has been updated. Good.