add the sources from apm-agent in to the final sources jar

[jackshirazi]

What does this PR do?

In 1.27.0 we restructured the subprojects so that all the main agent dependency management was moved from elastic-apm-agent to apm-agent, with no direct dependency on apm-agent from elastic-apm-agent. Instead it was made into a shaded dependency. This was to make the agent more isolated to reduce conflicts in classloading related to agent classes. An unnoticed side-effect of this was that the sources jar in elastic-apm-agent no longer contained the full agent sources.

To fix this, I tried a number of different configurations. Any configuration that directly added the apm-agent as a dependency caused the class jar file to contain the classes as well as the shaded classes, so those configurations failed. The shading plugin should have been able to create all the sources of the shaded dependencies, but although I tried really hard I couldn't find the magic combo that produced that. So I've gone with the brute force approach of just merging the sources from the apm-agent sources jar into the elastic-apm-agent sources jar. This has the advantage of being very simple, but the disadvantage of directly depending on a source jar file existing in another project.

Additionally this PR fixes the missing co.elastic.apm.agent.configuration -> co.elastic.apm.agent.premain.configuration that @JonasKunz noticed

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.asciidoc
  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • I have updated supported-technologies.asciidoc
  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.asciidoc

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• Start Time: 2023-03-21T16:45:05.059+0000

• Duration: 58 min 30 sec

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[JonasKunz]

Looks good to me, I do however have an idea on how to get rid of the direct apm-agent file based reference:

• use the maven-dependency-plugin execution in this pom above to explicitly copy the apm-agent sources JAR into the target folder (using the copy goal )
• use the copied sources-jar from the target folder instead of the file path ${project.basedir}/../apm-agent/target/

What do you think?

[jackshirazi]

I tried that a slightly different config, it requires the dependency to be declared as such which then would be included in the class files. Then I tried to exclude the dependencies after including them, then they don't show in the sources. So if I then do that and then copy the source jar, then do the merge it might work - but sounds like a very complex addition

[jackshirazi]

Actually I think I even tried it and couldn't get the sources if the classes were excluded, and couldn't prevent the classes from being the final class jar if they weren't even when I tried to exclude them also using the shade target. It was a really painful week

[jackshirazi]

nice change, thank you! Much better