Previously, if a call to doFilter in MetricFilter failed (i.e. it threw
an exception), it would be handled as if it had a response status of
200. This is because the servlet container was yet to handle the
exception and set the response status to 500.

This commit updates MetricFilter to assume that an exception thrown from
doFilter will result in a response with a status of 500. Strictly
speaking, even though the filter has highest precedence and will
therefore run last on the way back out, this may not always be the case.
For example, a custom Tomcat Valve could handle the exception and result
in a 200 response but that’s an edge case that’s into shooting yourself
in the foot territory.

Closes gh-2818