[DEBUG PR] Added debug logs to check why the spy doesn't work

[andsel]

Run with

SPEC_OPTS="-fd -P logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb" ./gradlew --rerun-tasks :logstash-core:rubyTests

It produces a console output like:

    DNADBG>> before - outer
    DNADBG>> log spy in before ListAppender [
        events=[], 
        messages=[INFO  [RSPEC Example LogStash::Setting::SettingWithDeprecatedAlias when only the deprecated alias is set logs a deprecation warning (from: `./logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb:124`)]: DNADBG>> Rspec test direct log
    ], 
       data=[], 
       newLine=false, 
       raw=false, 
       countDownLatch=null, 
       getHandler()=org.apache.logging.log4j.core.appender.DefaultErrorHandler@7faaa21f, 
       getLayout()=%-5p [%t]: %m%n, 
       getName()=LOG_SPY, 
       ignoreExceptions()=true, 
       getFilter()=null, 
       getState()=STARTED]
    DNADBG>> observePostProcess invoked on DeprecatedAlias: legacy.setting
    DNADBG>> spy from DeprecatedAlias.observePostProcess is: ListAppender [events=[], messages=[], data=[], newLine=false, raw=false, countDownLatch=null, getHandler()=org.apache.logging.log4j.core.appender.DefaultErrorHandler@7faaa21f, getLayout()=%-5p [%t]: %m%n, getName()=LOG_SPY, ignoreExceptions()=true, getFilter()=null, getState()=STARTED]
    DNADBG>> after - outer all appenders {LOG_SPY=ListAppender [events=[], messages=[], data=[], newLine=false, raw=false, countDownLatch=null, getHandler()=org.apache.logging.log4j.core.appender.DefaultErrorHandler@7faaa21f, getLayout()=%-5p [%t]: %m%n, getName()=LOG_SPY, ignoreExceptions()=true, getFilter()=null, getState()=STARTED]}
        logs a deprecation warning (FAILED - 2)

...
2) LogStash::Setting::SettingWithDeprecatedAlias when only the deprecated alias is set logs a deprecation warning
         Failure/Error: expect(log_spy.messages[0]).to include(deprecated_setting_name)
           expected nil to include "legacy.setting", but it does not respond to `include?`
         # ./logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb:125:in `block in <main>'
         # ./spec/spec_helper.rb:84:in `block in <main>'
         # ./logstash-core/lib/logstash/util.rb:43:in `set_thread_name'
         # ./spec/spec_helper.rb:83:in `block in <main>'
         # ./spec/spec_helper.rb:76:in `block in <main>'
         # ./vendor/bundle/jruby/3.1.0/gems/logstash-devutils-2.6.2-java/lib/logstash/devutils/rspec/spec_helper.rb:47:in `block in <main>'
         # ./lib/bootstrap/rspec.rb:36:in `<main>'
...
rspec ./logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb:124 # LogStash::Setting::SettingWithDeprecatedAlias when only the deprecated alias is set logs a deprecation warning

Problem statement

While in the setup section of the test the ListAppender (in state STARTED) works, grabbing the artificial log line, it doesn't catch any log that is created in the DeprecatedAlias,