[docs] Log4j configuration needs to be better documented

[devinbost]

We currently don't have much documentation around how to configure Log4j debugging for Pulsar, and the log4j docs are confusing. For example, I'm trying to enable DEBUG logging for Pulsar, but we're having trouble getting debug statements to show up in our rolling log. The Pulsar docs don't provide sufficient explanation around how to do this.

[devinbost]

For example, the following Log4j file doesn't emit debug statements for permits from ConsumerImpl, Consumer, or PersistentDispatcherMultipleConsumers

log4j2.yaml.txt
(Ignore the .txt extension. That was just to enable uploading it into the Github issue.)

[devinbost]

I was able to get debug statements after hard coding "debug" in place of "${sys:pulsar.log.level}" in this part of the log4j2.yaml file:

Root:
      level: debug
      additivity: true
      AppenderRef:
        - ref: "${sys:pulsar.log.appender}"
          level: "${sys:pulsar.log.level}"
        - ref: Prometheus
          level: info

However, I haven't been able to get debug logging on the functions.

[codelipenghui]

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