Fix: Sidekiq workers delete each other's metrics

What does this MR do?

Fixes #336311 (closed)

When we moved the logic that wipes the Prometheus metrics dir out of the Rackup file and into the initializer, all Sidekiq workers would call this and potentially enter a race condition where they deleted each other's database files.

Since config.ru is only executed by Puma, and since this call is guarded by only running for the primary, this should not happen anymore.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

Tested with gitlab-ee:52edbc6e312c826c5b47f5cf65d60adee770b031 and verified that all prometheus db files remained intact across restarts:

root@local:/# ll /dev/shm/gitlab/sidekiq
total 10856
drwx------ 2 git  root     280 Jul 20 12:53 ./
drwxr-xr-x 4 root root      80 Jul 20 12:33 ../
-rw-r--r-- 1 git  git     4096 Jul 20 12:53 counter_sidekiq_0-0.db
-rw-r--r-- 1 git  git     4096 Jul 20 12:53 counter_sidekiq_1-0.db
-rw-r--r-- 1 git  git     8192 Jul 20 12:54 counter_sidekiq_2-0.db
-rw-r--r-- 1 git  git     8192 Jul 20 12:54 gauge_all_sidekiq_0-0.db
-rw-r--r-- 1 git  git     8192 Jul 20 12:54 gauge_all_sidekiq_1-0.db
-rw-r--r-- 1 git  git     8192 Jul 20 12:54 gauge_all_sidekiq_2-0.db
-rw-r--r-- 1 git  git     4096 Jul 20 12:53 gauge_max_sidekiq_0-0.db
-rw-r--r-- 1 git  git     4096 Jul 20 12:53 gauge_max_sidekiq_1-0.db
-rw-r--r-- 1 git  git     4096 Jul 20 12:53 gauge_max_sidekiq_2-0.db
-rw-r--r-- 1 git  git  4194304 Jul 20 12:54 histogram_sidekiq_0-0.db
-rw-r--r-- 1 git  git  4194304 Jul 20 12:54 histogram_sidekiq_1-0.db
-rw-r--r-- 1 git  git  4194304 Jul 20 12:54 histogram_sidekiq_2-0.db

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.