We were accidentally deleting metrics the exporter
server exported about itself. Actual sidekiq worker
metrics were not affected.

This happened because we would wipe the metrics dir
whenever sidekiq_0 starts up, but this happens after
the metrics server starts, so the worker was deleting
those existing metrics.

Changelog: fixed

The plan for RSpec 4.0 is to disable monkey patching, reference:
https://rspec.info/blog/2013/07/the-plan-for-rspec-3/#zero-monkey-patching-mode
https://relishapp.com/rspec/rspec-core/docs/configuration/zero-monkey-patching-mode

This commit stops using RSpec monkey patching in several specs.

After moving the multiproc dir cleanup into `config.ru`:`warmup`, we
stopped cleaning Sidekiq metrics dir which is not correct.
This MR intended to fix that. More details:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31668