Send Logstash Metrics via OTLP

[estolfo]

Release notes

What does this PR do?

This PR adds a new way to collect and export metrics from Logstash. It adds the Java OTel SDK that is used to register and collect metrics and export them to an OTLP endpoint.

As many of the existing metrics available in Logstash are exported as possible. The only metrics not exported are FlowRate metrics.

ToDo:

• add documentation
• Add more detailed PR description

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

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

• [ ]

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[mergify]

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

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[elasticmachine]

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