Previously, if an exception was thrown during request handling after
the response had been committed, i.e. after the status and headers
had been written, the metrics filter would assume that it was a 500
response. This was potentially inaccurate as the status had already
been sent to the client and before the exception was thrown and it
may have been something other than a 500.

This commit updates MetricsFilter so that it will use the
status from the response if the response has been committed even when
an exception is thrown.

Closes gh-7277