Initial dashboard for MR lead time metric

Vladimir Glafirovrequested to merge
mr_lead_time into master

Merge request lead time dashboard. Initial version.

Dashboard snapshot: https://dashboards.gitlab.net/goto/XkQ-mz-4z?orgId=1

Related issue: gitlab-com/gl-infra/delivery#2845 (closed)

Edited by Vladimir Glafirov

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