Provide a request attribute for storing exception info for tagging purposes

[bclozel]

As part of #23795, we've found that errors handled by the web infrastructure are inconsistently tagged by our metrics support: an "exception" tag is added too broadly, and only in some cases.

Since #23795 will remove that behavior by default, we'd like to provide a way for developers to still get that behavior on an opt-in basis.

We should provide a request attribute for both MVC and WebFlux that developers can use and set to the exception being handled by the application code. Right now we have a similar (yet private) mechanism in both implementations of DefaultErrorAttributes that stores the exception as early as possible in an attribute.

Developers could then opt-in and store the exception if they wish to:

@ExceptionHandler(MyException.class)
public String myExceptionHandler(HttpServletRequest req, MyException exc) {
  req.setAttribute(ErrorAttributes.EXCEPTION, exc);
  return " fallback"
}

In this case, the metrics infrastructure would add the "exception" tag to the recorded metric.