Cherry-pick #21693 to 7.x: Update system/socket dataset to support config reloading

[adriansr]

Cherry-pick of PR #21693 to 7.x branch. Original message:

What does this PR do?

This updates the system/socket dataset so that it behaves correctly when run under config reloader.

When a dataset is run from a static module definition in auditbeat.yml:

• Factory method New is called.
• (instance).Run is called.

When the dataset is configured in the context of configuration reloading:

• Factory method New is called to validate that the config is OK. The returned instance is discarded without its context being closed.
• Factory method New is called again.
• (instance).Run is called for the instance returned by the second New.

Also, if reload.enable is true, everytime the configuration file changes:

• The metricset context is closed.
• A new instance is created using New.
• (instance).Run is called.

This behavior didn't play well with a dataset with complex initialization (can take up to 20s to start) and that uses global OS resources (kprobes) that need to be released before a new dataset can be instantiated. There's no mechanism for the reloader to wait for a metricset's termination.

This updates the dataset factory method to keep track of an already running instance. Also it checks if the config has changed to prevent initializing two metricsets at startup.

Why is it important?

Currently Auditbeat will fail with a cryptic error when the socket dataset is started using config reloading. See #20851.

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.

How to test this PR locally

See the linked issue.

Related issues

• Closes Auditbeat system/socket fails when started by config reloading #20851

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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• Build Cause: [Pull request #22730 opened]

• Start Time: 2020-11-24T10:14:50.965+0000

• Duration: 25 min 38 sec

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[leehinman]

LGTM

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• Build Cause: [Pull request #22730 updated]

• Start Time: 2020-11-24T18:11:08.304+0000

• Duration: 22 min 22 sec

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Passed	228
Skipped	33
Total	261