Previously, each 5xx response to a request that used a path variable
would result in a metric being recorded that contained the path
variable. Therefore, if a different path variable was included in each
request, a new metric would be recorded for each request. This is
problematic as it can lead to the metrics being flooded with unwanted
entries.

This commit updates MetricsFilter to treat 5xx responses sent before
mapping has occurred in the same way as 4xx and redirect responses.
A single metric, counter.status.500.unmapped, is now used.

Closes gh-4377