Fix metrics related race condition in sidekiq-cluster

With !83407 (merged) merged, there is a slight chance that the metrics directory gets wiped after workers have started up, which would result in their metrics missing.

I don't think we are actually running into this problem, since cleanup happens right after forking, and it takes Sidekiq workers several seconds to boot and start emitting metrics. But in theory, it could happen.

Not adding a changelog since the problem was introduced and fixed in 14.10.

Related to #356505 (closed)