Summary

EuiLoading needs to better communicate it's state to assistive tech. We can improve the component in two ways:

1. First, add role="progressbar" and a default aria-label value to the loading state
2. Also, accept a child to handle properly setting aria-busy

Details

Doing the first, improves the existing usage of the component. Doing the second, is a non-breaking API change that should better handle a more ideal UX.

For existing implementations, or for those that can't/don't want to upgrade to the new pattern, we should provide better documentation around aria-label and aria-busy

Documentation on aria-label should largely ask that something better than "loading content" be passed in.

Documentation on aria-busy should talk about where to put it (around the loading content) and give warnings on how it works (that it treats all children as aria-hidden) so devs don't overuse it.

In addition to accepting children to handle the aria-busy attribute setting, it'll need to also accept an isLoading prop (which is required if children are passed in).