Enable Sidekiq LB by default

Release notes

Introduced in GitLab %14.1, load balancing for Sidekiq is enabled by default in %14.2.

Documentation: https://docs.gitlab.com/ee/administration/database_load_balancing.html#load-balancing-for-sidekiq

Problem to solve

In Allow Sidekiq jobs to use read-only database replicas we introduced the ENV variable ENABLE_LOAD_BALANCING_FOR_SIDEKIQ in order to enable LB for sidekiq.

Then we proceeded by setting ENABLE_LOAD_BALANCING_FOR_SIDEKIQ=true on production.

We should enable this behavior by default and remove the ENV variable.

• I prepared Enable Sidekiq load balancing by default
• I prepared MR for k8s: Remove load balancing for sidekiq env variable
• We also need to remove this on the chef-repo: https://ops.gitlab.net/gitlab-com/gl-infra/chef-repo/-/blob/master/roles/gstg-base-be-sidekiq.json#L27