feat: consume APM configuration over the elastic-agent control protocol

[kruskall]

Motivation/summary

Consume APM configuration over the elastic-agent control protocol. Pin elastic-agent-client version to 7.6.0 until beats is updated.

Create a new instrumentation package to retrieve the tracer dynamically. It might need some changes in go-docappender and other packages to allow updating the tracer.

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

Testing is a nightmare and can't be easily performed. We can't run elastic-agent locally and the ea-managed apm-server system test is broken. Current approach is to start the ea-managed smoke test, manually install elastic-agent, upload apm-server binary to replace the apm-server component and restart elastic-agent.

Related issues

[mergify[bot]]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏 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.

[cmacknz]

Testing is a nightmare and can't be easily performed. We can't run elastic-agent locally

Best solution to this is a VM, I like https://multipass.run/ especially if you are a Mac. Makes dealing with Ubuntu VMs as easy as containers.

[v1v]

run docs-build

[kruskall]

This requires changes in beats, I'll close this for now