Fix statsd app logging
Fix statsd app logging

The metric part of this probably never worked because `@field.status` is a typo and should've been `@fields.status`, but the timers will have stopped working for Whitehall (and other apps?) in May 2017 when logstasher was upgraded to version 1, removing the `@fields` prefix from the JSON logging.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet