[Metricbeat] Add process_list metricset to MySQL module

[sayden]

What does this PR do?

Adds process list information to the MySQL module as a separate metricset.

Why is it important?

This is the kind of information that is very useful for some type of MySQL users but not so much to other so it's better to have it in an isolated Metricset. Allows to see the list of processes in MySQL by user, db, and the query that is being executed, as well as the connection ID to kill the process if necessary.

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
• [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Implements / Closes need help to debug why Packetbeat not capturing postgresql #123

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b feature/mb/mysql/show_process_list upstream/feature/mb/mysql/show_process_list
git merge upstream/master
git push upstream feature/mb/mysql/show_process_list

[mergify]

This pull request does not have a backport label. Could you fix it @sayden? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[jlind23]

@sayden - Closing this one as there were no activity for a while