Fix/log4j routing to avoid create spurious file

[andsel]

Release notes

What does this PR do?

This PR avoid to create pipeline log appender when the log event doesn't contain the pipeline.id

Why is it important/What is the impact to the user?

Avoid that the user see files name like 'pipeline_${ctx:pipeline.id}.log' which are empty

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

• in config/log4j2.properties find the line

  logstash/config/log4j2.properties

  Line 57 in cafbf03

  appender.routing.pipeline.policy.size = 100MB

• change the size from 100MB to 1KB
• enable pipeline.separate_logs in config/logstash.yml
• run Logstash with following pipeline.conf bin/logstash -f pipeline.conf --debug
• check in logs/ directory the not existence of a file named pipeline_${ctx:pipeline.id}.log. On master it's present before this PR, in the branch of this PR that file is not created.

pipeline.conf

input {
  java_generator {
    lines => [
      "line 1",
      "line 2",
      "line 3"
    ]
    eps => 1
  }
}

output {
  stdout {
    codec => rubydebug
  }

Related issues

Use cases

Screenshots

Logs

[kaisecheng]

I have tested the branch, confirmed there is no pipeline_${ctx:pipeline.id}.log and rotation work properly. The master branch and the testing branch is throwing

org.reflections.ReflectionsException: could not create class object from file org/logstash/plugins/plugin_aliases.yml

which is not in the scope of this issue.
Nice integration test 👍 . LGTM