Manage reviewers load better via reviewer queues
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### Problem to solve
Currently, we don't have a reliable way to optimally distribute the workload between all reviewers and maintainers across all teams' MRs during the release. Often, a particular reviewer is overloaded with MRs to check, burning them out and from another hand making him a bottleneck.
### Target audience
GitLab team and contributors
### Further details
When an MR is ready, it's authors usually looks for the reviewer or maintainer [at the projects page](https://about.gitlab.com/handbook/engineering/projects/) of the handbook. But it does not tell anything about the current reviewer's workload. A reviewer, when assigned, starts looking someone less burdened and re-assign to another reviewer, and this goes on.
### Proposal
1. Create a the publicly visible single source of truth telling how many MRs are currently being reviewed by a particular reviewer or maintainer
1. This SSOT should be either editable by reviewers (maintainers) or automatically react to the current state of things at GitLab.com
1. Set a limit of MRs allowed for simultenous review by a single reviewer
### What does success look like, and how can we measure that?
Decrease of the time spent by MRs in Review state according to [GitLab CE](https://gitlab.com/gitlab-org/gitlab-ce/cycle_analytics) and [GitLab EE](https://gitlab.com/gitlab-org/gitlab-ee/cycle_analytics) Cycle Analytics at GitLab.com
### Links / references
Automatic display of current workload of reviewers could be made possible after adoption of [multiple assignees per MR](https://gitlab.com/gitlab-org/gitlab-ee/issues/2004).
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