Support for setting the application class loader for a transaction via the public api

[tobiasstadler]

What does this PR do?

Implements the solution suggested in #2369

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

[github-actions]

👋 @tobiasstadler Thanks a lot for your contribution!

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• Start Time: 2022-02-15T09:39:24.552+0000

• Duration: 54 min 54 sec

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[felixbarny]

I've added some changes, let me know if this works for you.
LGTM, but I'd like to wait for @eyalkoren's feedback before merging

[felixbarny]

run elasticsearch-ci/docs

[felixbarny]

run elasticsearch-ci/docs

[tobiasstadler]

I've added some changes, let me know if this works for you.

Much better that what I did! Thank you for your support.

Yes, service name/version and MDC work for me.

[felixbarny]

run elasticsearch-ci/docs

[eyalkoren]

I actually intend to get rid of TraceContext#applicationClassLoader as part of #2428 ...
We don't need this reference anymore and it's always a good idea to store one less weak reference (and any CL reference).
Since you are actually only interested in the implicit side-effect of TraceContext#setApplicationClassLoader, which is to override the service info for the transaction, the API seems inappropriate to me. Calling it overrideServiceInfoFor(ClassLoader) would be better (the javadoc would how the provided ClassLoader is used).

Generally speaking, I would prefer a cleaner and more generic API such as overrideServiceInfo(String serviceName, String serviceVersion). If extracting the service version and name are simple enough- just implement that instead of relying on the automatic discovery. If relying on the automatic discovery is important, I assume this is mostly relevant for external plugins (not for other usages of the API), so we can expose ElasticApmTracer#getServiceInfo(ClassLoader) through the sdk by making it part of the co.elastic.apm.agent.impl.Tracer interface. WDYT?

[tobiasstadler]

Hm, I find it quite inconvenient to (always) have to to call getServiceInfo before I can call overrideServiceInfo. What about something like Transaction#withServiceInfoForClassCloader(ClassLoader) (or maybe use instead of with or another better name) or #2369. What do you think?

@eyalkoren I will implement your suggestion in the next days

[tobiasstadler]

@eyalkoren Please see #2451 for my implementation of your suggestion

[tobiasstadler]

#2451 is the preferred api