Consider making `id` optional.

Element timing entries returns both an element and an id for its timings.

LCP timing entries also have id, but typically juts "".

Container Timing right now, same as element timing, uses "id assignment" as the opt-in mechanism. But often this just leads to generating random identifiers for many use cases, or name clashes.

Consider an alternative:

• Perhaps any containertiming-nesting strategy should suffice to opt-in for container-timing (nesting implies this is, itself, a container).
• elementtiming="id" could still be used to assign an explicit id
• element is returned to help identify the container, but might be null.
  • We can solve for that case by including a querySelector value (this would be a separate feature request for all element timings, a common request), (or the developer can assign an id)
• This also removes the need to have "default" strategies for nesting

Start by reviewing the Container Timing behavior described in the issue, focusing on containertiming-nesting, elementtiming="id", and the returned element and id fields. Done means reaching an agreed specification for optional ids, opt-in behavior, null elements, and nesting strategies.