Capture db.instance for Cassandra

[SylvainJuge]

What does this PR do?

• capture keyspace for cassandra as db.instance (relates to Spec: services dependencies granularity NoSQL apm#654)
• ad some convenient assertions for spans + some refactor

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 n/a
  • I have made corresponding changes to the documentation n/a

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-08-01T08:39:23.158+0000

• Duration: 51 min 15 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3081
Skipped	36
Total	3117

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark tests.

• run jdk compatibility tests : Run the JDK Compatibility tests.

• run integration tests : Run the Agent Integration tests.

• run end-to-end tests : Run the APM-ITs.

• run windows tests : Build & tests on windows.

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

[github-actions]

/test