Fix EuiCodeBlock not copying updated content

[elizabetdev]

Summary

Closes #3321.

This PR tries to fix the bug with the EuiCodeBlock does not seem to update the value to copy when the content changes. To fix this bug, a unique key was added to the EuiInnerText. Every time the content changes a new key is generated so this makes possible to copy the new content.

Checklist

• [ ] Check against all themes for compatibility in both light and dark modes
• [ ] Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• [ ] Added documentation examples
• [ ] Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3351/

[elizabetdev]

jenkins test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3351/

[chandlerprall]

Thanks for taking this!

I was curious why EuiInnerText's mutation observer wasn't catching these changes, and a new one had to be created every time - wanted to know if this was somehow unique to code blocks or a bug in useInnerText. Turns out its mutation observer configuration is missing childList: true.

Had to dig into the difference between childList and subtree: subtree applies the configuration & observer to the existing elements, while childList instructs the observer to update when elements are added or removed from the tree. useInnerText needs both.

[elizabetdev]

Thanks, @Chandler. I've just updated the code with what you suggested. I tested and it's working!

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3351/

[chandlerprall]

Thanks! Change LGTM; pulled & tested locally