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

…oogleCloudPlatform#9340)

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.

run/logging-manual/src/main/java/com/example/cloudrun/App.java

@@ -58,6 +58,8 @@ public static void main(String[] args) {
           // -- End log correlation code --
 
           // Create a structured log entry using key value pairs.
+          // For instantiating the "logger" variable, see
+          // https://cloud.google.com/run/docs/logging#run_manual_logging-java
           logger.error(
               "This is the default display field.",
               kv("component", "arbitrary-property"),