A core component keeps restarting making the metrics unavailable for fetch. No metrics are lost during those restarts. We will take actions to fix this issue in the upcoming days.
An action was taken at 02:30 UTC (2018-11-21) which has successfully fixed this issue. This is only temporary though.
A permanent fix will be applied later today, which will require a downtime of that component.
EDIT 2018-11-21 16:50 UTC: The permanent fix is delayed to tomorrow, 2018-11-22.
EDIT 2018-11-22 10:40 UTC: The fix will be applied at 10:50 UTC, this will require at least one restart of that component which will lead to an unavailabiliy of Metrics for about 20 minutes.
EDIT 2018-11-22 11:25 UTC: Metrics are back since 11:08 UTC. Incident over.