Added Instrumentation for ServletContextListener#contextInitialized

[tobiasstadler]

What does this PR do?

Improves service name and version auto detection

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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• Reason: null

• Start Time: 2022-02-03T12:58:52.278+0000

• Duration: 48 min 59 sec

• Commit: fc5fbe4

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Test	Results
Failed	0
Passed	2490
Skipped	16
Total	2506

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[felixbarny]

run elasticsearch-ci/docs

[tobiasstadler]

Thank You!

[felixbarny]

Thank you too!

Do you still have a need for Tomcat, JBoss/WildFly specific instrumentations to init earlier and to make it work for JMS? I think now that we have more fundamentals in place, such asServletServiceNameHelper as well as generic instrumentation via the Servlet API, I'm also more comfortable with adding container-specific instrumentations. That's because if they don't work in a new version of the container, we at least have a fallback that's good enough for most cases.

To make these container-specific instrumentations even more lightweight, I'd just add them in the apm-servlet-plugin. For example under co.elastic.apm.agent.servlet.init or co.elastic.apm.agent.servlet.servicename.

[tobiasstadler]

Yes, at the moment I still prefer having the "correct" service name/version early in the process.