[8.15] fix: Add wrapper around libbeat instrumentation to propagate missing tracer config (backport #13653)

[mergify]

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

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This is an automatic backport of pull request #13653 done by [Mergify](https://mergify.com).