Minimize archival lag w.r.t. upstream code hosting platforms

• Quantify and monitor in real-time the lag, especially for major platforms (GitHub, GitLab.com, etc.) - #4130

• Improve ingestion efficiency (optimize loaders, especially the Git loader, optimize scheduling policies) - #2207

• Make lag monitoring dashboards easy to find (for decision makers) - #4133

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Migrated from T4080 (view on Phabricator)