Adding a simple Cells dashboard

Andy Hohennerrequested to merge
cells_performance into master

Context

Working on recreating the performance dashboard for Cells, this MR includes a simple version to test creating the snapshots: https://gitlab.com/gitlab-com/runbooks/-/blob/cells_performance/dashboards/cells/main.dashboard.jsonnet?ref_type=heads

This is for troubleshooting https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/3925

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