gitaly: Add server-side backup dashboards

Issue: gitlab-org/gitaly#5358 (closed)

Modifies the Main and Host Detail Gitaly dashboards to include metrics on server-side backups. Links to test dashboards:

• https://dashboards.gitlab.net/dashboard/snapshot/GYxExAkr8h3D4EXh4EoeWBFT9JndAh61 - gitaly: Overview
• https://dashboards.gitlab.net/dashboard/snapshot/PFJ8cPhWRPDI8sAkOpFYA576Jks8Cex2 - gitaly: Host Detail

Note that the panels won't have any data present yet, as server-side backups are not yet used for .com.