monitoring: enable tracing self instrumentation in APM Server

[1pkg]

Motivation/summary

This PR addresses the issue #14230 and enables self instrumentation in APM Server.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

For the detailed steps to test this changes in self-hosted APM Server refer to #13514 (comment)

Related issues

#14230

[mergify]

This pull request does not have a backport label. Could you fix it @1pkg? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[marclop]

I think this change needs a changelog entry

[1pkg]

While testing the changes manually, I discovered a bug in EA tracing sampling changes where sampling_rate is formatted as a binary (hex) with exponent instead of decimal literal without exponent, this breaks subsequent call in apm-agent-go when tracer is initialized with a custom sampling rate.

[1pkg]

Raised an issue in the agent repo elastic/elastic-agent#5711

[axw]

Thanks @1pkg!

[1pkg]

Can't be merged yet due to the Cloud configuration missing sensible default for sampling_rate.