fix: Add wrapper around libbeat instrumentation to propagate missing tracer config

[1pkg]

Motivation/summary

This PR addresses the issue by adding a thin wrapper around libbeat instrumentation to propagate missing tracer config from elastic-agent via environmental variables.

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

Follow instructions in Building an Elastic Agent container image with a locally built APM Server and Running an Elastic Agent container with a locally built APM Server section in TESTING.md, update the relevant agent.monitoring sections in elastic-agent.yml to see the config are propagated down to tracing correctly.

Related issues

#13597
#11381
#13514

[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.

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

[axw]

LGTM. Would be good (but perhaps not critical) to have a functional test for this. WDYT? Maybe open an issue to add a system test?

[marclop]

Looks great! Thanks for adding the test.

[carsonip]

thanks for the unit test! left a few nits