Prior to this commit, some exceptions handled at the controller or
handler function level would:

* not bubble up to the Spring Boot error handling support
* not be tagged as part of the request metrics

This situation is inconsistent because in general, exceptions handled at
the controller level can be considered as expected behavior.
Also, depending on how the exception is handled, the request metrics
might not be tagged with the exception.
This will be reconsidered in gh-23795.

This commit prepares a transition to the new situation. Developers can
now opt-in and set the handled exception as a request attribute. This
well-known attribute will be later read by the metrics support and used
for tagging the request metrics with the exception provided.

This mechanism is automatically used by the error handling support in
Spring Boot.

Closes gh-24028