SLO for end-to-end time to initial code review for MRs from leading organizations

Why is this change being made?

We already have an SLO to perform an initial code review for MRs from Leading Organizations within 4 business days once the MR is triaged. This SLO is useful as a guideline for code reviewers. It helps them plan their workload to get the code reviews done in the expected time.

This SLO seems less meaningful to Leading Organizations and other contributors from the wider community. I assume contributors from outside of GitLab care more about the time from when their code is ready to review to when they get the initial review.

All that is missing to get to the latter metric is an SLO to triage MRs from Leading Organizations and assign a reviewer within a certain period of time. I am putting in 3 days primarily as a conversation starter.

This is still an SLO, not an SLA.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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