fix(webflux): fix NoSuchMethodError when using spring framework 7 with webflux

[jihad-bouhaiji]

What does this PR do?

This PR fixes an issue with the experimental support for Spring framework 7 that was added PR. When running the agent, it will throw a NoSuchMethodError exception due to the HttpHeaders.entrySet method. This is due to HttpHeaders no longer implementing the MultiValueMap contract.
I have changed the copying of header to only use API's that are still available across version 5, 6 and 7.
The test co.elastic.apm.agent.springwebflux.AbstractServerInstrumentationTest#dispatchHello covers this fix.

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.next-release.md
  • 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.next-release.md
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.next-release.md
  • 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.md
  • 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.next-release.md

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[SylvainJuge]

Can you try to add an explicit test that allows to reproduce it ? Is the issue happening with the version that is being used for compilation ? if yes, then a simple unit test should be enough. If not, then do you have a sample application that allows to reproduce the issue ?

[SylvainJuge]

[nit] reword the comment to explain the "why".

Suggested change

	// This is the only way I could find to make a multiValueMap without having access
	// to the api from spring framework 7 that will work across versions 5, 6 and 7
	// This allows to to make a MultiValueMap without having access
	// to the api from spring framework 7 that will work across versions 5, 6 and 7

[SylvainJuge]

[minor] we don't need a parallelStream here as only a few values are expected, so it is probably better to use the non parallel one.

[jihad-bouhaiji]

I added a module apm-spring-webflux-plugin-spring7 and apm-spring-webflux-testapp-spring7 that tests the same flows that we have for Spring 6 in apm-spring-webflux-plugin. That will catch any breaking api mismatch across 5, 6, and 7.\

I do have a question.
It turns out that the api for HttpHeaders.get() has also changed in springframework 7. So I changed it to use getValuesAsList. However for the test co.elastic.apm.agent.springwebflux.AbstractServerInstrumentationTest#hello, it expects a comma seperated string for the header "Accept" instead of a List of Strings.
I have changed the test to reflect that the headers are copied as a list instead of one String, but i want to double check with you if that was okay, or if i should parse all multi value headers into one comma seperated string?