Implement ResourceUsageLimit sidekiq server middleware
What does this MR do and why?
This MR implements a ResourceUsageLimit Sidekiq server middleware. It is behind a feature flag. The middleware uses a wrapper over Gitlab::ApplicationRateLimiter to track resource usages. The resource in question refers to any key tracked in the SafeRequestStore such as cpu, redis, database, gitaly durations/counts etc. But for this MR and gitlab.com's short-term scalability concerns, we focus on db_main_duration_s and db_ci_duration_s.
The resource usage limits are hardcoded with some sane values based on gitlab-com/gl-infra/scalability#3859. Note that the follow-up tasks will be to enable users to configure the limits using ApplicationSettings (gitlab-com/gl-infra/scalability#3920).
Note: This MR was broken out from !168193 (closed) and improved.
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