fix missing method in ruby pipeline

[kaisecheng]

Release notes

Fixed ruby engine pipeline crash during shutdown

What does this PR do?

This PR adds the missing method worker_threads_draining? to ruby pipeline which is added in #13934 to java pipeline for log msg improvement.

As ruby engine is deprecated in Logstash 8, this method is added for backward compatibility.

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

Logstash runs in ruby engine and enables queue.drain got NoMethodError during shutdown. The ruby pipeline crashes.

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

• test it manually

How to test this PR locally

1. logstash.yml
   queue.type: persisted
   queue.drain: true

2. run the pipeline with ruby engine bin/logstash --java-execution false

input { stdin {} }
output {
  tcp {
    host => "localhost"
    port => "3000"
    ssl_enable => false
  }
}

3. no need to start tcp server
4. send event to stdin, eg "asdf" to accumulate events in PQ
5. send SIGINT to shutdown logstash
6. Logstash should not show NoMethodError

Related issues

Relates #13934
Closes #15010

[andsel]

LGTM