Add apm trace sampling

[pchila]

What does this PR do?

This PR adds the possibility to specify apm sample rate in elastic agent APM configuration

Why is it important?

Specifying a sample rate allows controlling the ingestion rate, reducing overhead and storage requirements when self-monitoring is active

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
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

• Closes Add missing tracing sampling APM config section #5211

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[mergify]

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

• backport-v./d./d./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.

[mergify]

backport-v8.x has been added to help with the transition to the new branch 8.x.

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b add-apm-trace-sampling upstream/add-apm-trace-sampling
git merge upstream/main
git push upstream add-apm-trace-sampling

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[blakerouse]

More testing code then actual change code. I appreciate the attention to coverage in this change, well done!

[andrzej-stencel]

I'm not seeing any validation for the value of SamplingRate. Should there be any? I assume the value of SamplingRate should be a positive number lower than 1?

[pchila]

I'm not seeing any validation for the value of SamplingRate. Should there be any? I assume the value of SamplingRate should be a positive number lower than 1?

According to the APM documentation https://www.elastic.co/guide/en/apm/agent/go/current/configuration.html#config-transaction-sample-rate it should be a float between 0.0 and 1.0

If we have to add a validation also on agent side we would need to rely on ucfg module as we do for all the rest of the config although I would prefer to leave that to any validation performed in the apm module

[elastic-sonarqube]

Quality Gate failed

Failed conditions
20.0% Coverage on New Code (required ≥ 40%)

See analysis details on SonarQube

[ycombinator]

SonarQube is complaining about lines in run.go being not tested; code in this file is not easy to unit test. Force merging...