Fix publishing-api queue latency check
Fix publishing-api queue latency check

I had accidentally used the `enqueued` metric, not the `latency`
metric.  So this would instead alert if there were more than 45
documents in the queue, which we surpass all the time.

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