Increase history of the blackbox exporter.

The blackbox exporter is currently configured with --history.limit=1000. It's not clear (to me) what the number represents, my guess is lines of log kept in memory (can be retrieved via the web UI).

When looking into the failure in the linked incident, I was unable to figure out what led to failing probes, because the history had already been rotated out.

Since the process currently uses 60 MByte of RSS I think we can easily increase this limit by a factor of 10–20, which would give us a bit more history for debugging. The machine has ~16 GByte of memory, of which it only uses ~2.5 GByte. We could put this memory to good use instead of having it sit idle.

This ticket was created from INC-5648 and was automatically exported by incident.io 🔥