Added support for collecting statistics about dropped exit spans

[tobiasstadler]

What does this PR do?

Fixes #2084

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!

It may take some time before we review a PR, so even if you don’t see activity for some time, it does not mean that we have forgotten about it.

Every once in a while we go through a process of prioritization, after which we are focussing on the tasks that were planned for the upcoming milestone. The prioritization status is typically reflected through the PR labels. It could be pending triage, a candidate for a future milestone, or have a target milestone set to it.

[apmmachine]

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• Start Time: 2022-03-14T14:46:15.253+0000

• Duration: 48 min 33 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2749
Skipped	16
Total	2765

💚 Flaky test report

Tests succeeded.

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[jackshirazi]

@elasticmachine run elasticsearch-ci/docs

[tobiasstadler]

Thank You!