Fix flaky webflux transaction name

[SylvainJuge]

What does this PR do?

Fixes flaky transaction name when Webflux is used with Servlet transaction.

The underlying issue is that the transaction name is set in Webflux thread just after the transaction ends, which makes the Servlet instrumentation end the transaction and sometimes use a default name.

In order to fix this, I've just moved the part that sets the transaction name just before the transaction is being ended, which allows to make sure that the Servlet transaction won't be ended until the nested onError or onComplete method is called.

When the transaction is created and ended by Webflux without servlets, this does not change anything.

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix: they used to fail a lot, now they run without error.

[eyalkoren]

👍

[github-actions]

/test

[ghost]

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Build stats

• Start Time: 2022-07-05T13:30:00.102+0000

• Duration: 49 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3053
Skipped	36
Total	3089

💚 Flaky test report

Tests succeeded.

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