monitoring: add more panels to Grafana repo-updater dashboard

Created by: unknwon

Raw notes:

• Is syncing health?
  • Data points for syncing completes, how much time and how many repos for each sync
  • Schedule queue, how big is it
  • What do we need if customer thinks repos are not updating
  • Are repos being refreshed appropriately on gitservers?
  • Number of update requests to gitservers
• Bonus: How is indexing doing on Zoekt?

https://github.com/sourcegraph/customer/issues/106