GitLab scaling above 2k user ref architecture

[jacobbmay]

Our current configuration is based on GitLab 2k user architecture. Starting with 3k and above, it recommends HA and/or clustered versions of services like Postgres and Gitaly. We need to support scaling above the 2k user reference architecture.

Tasks

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1. Investigate & Enable Sharded Gitaly #213
   7 of 7
   customer-support documentation enhancement top-level +4
   
2. Configure gitaly with recommended resiliency settings #221
   1 of 2
   top-level +1
   
3. Investigate HA postgres configuration with NDB #189
   top-level +1
   
4. Enable 2k users by configuring pgbouncer in cluster #219
   top-level +1
5. Enable 3k user count via valkey cluster & potentially cache #220
   top-level +1
   
6. Additional GitLab load testing for higher target user counts #191
   top-level +1