Previously, MetricsFilter would treat async requests the same as
sync requests and would record their response status as soon as the
request had been processed by the filter chain. This would result in a
200 response being recorded and the actual response status produced by
the async processing being ignored. Furthermore, the time that was
recorded for the request would not include the time take for any async processing.

This commit updates MetricsFilter to check whether or not an async
request has been started before it records the metrics for the request.
In the event of an async request having been started no metrics are
recorded. Instead, the StopWatch is stored in a request attribute that
is then retrieved during the filtering of the request’s async
dispatch(es). Once the async processing has completed (isAsyncStarted()
returns false), the StopWatch is stopped, removed from the request’s
attributes and the metrics for the request are recorded.

Closes gh-4098