When a worker runs for the first time, it will have
sidekiq_jobs_completion_seconds_count set to 1 (because it was just
completed). But this is wrong in Prometheus terms, because there was no
previous value of 0, and so the rate of change will also be 0.

To work around this, we initialise this bucket for all workers that can
be run by the current Sidekiq process before processing the first job.
This way we can calculate rates correctly for infrequent workers.