Add graphs for CustomersDot Sidekiq SLIs

Etienne Baquérequested to merge
eb-sidekiq-endpoints-customersdot into master

Related to gitlab-org/customers-gitlab-com#4767 (closed)

Grafana snapshot (after removal of previously set sidekiq_jobs definition): https://dashboards.gitlab.net/dashboard/snapshot/RsxuQRCLT8VaBK0oUyzIqX9E0yebUFGv?orgId=1

Edited by Etienne Baqué

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