add hit-ratio panel for urlcache in registry storage dashboard

Pawel Rozlachrequested to merge
prozlach/add_urlcache_hit_ratio into master

Rendered playgroung dashboard: https://dashboards.gitlab.net/goto/Ns_HR__Hg?orgId=1

This MR add the hit-ratio panel to urlcache for container-registry storage dashboard.

Issue: gitlab-org/container-registry#1622 (closed)

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