Add pack objects panels to Gitaly dashboards

This MR adds pack-objects metrics to Gitaly dashboards. The list of metrics includes:

• Pack objects cache operations: hit/misses/bytes served/bytes generated.
• Pack objects concurrency.
• Pack objects queuing and dropped commands.

Those metrics are useful for rolling out the new Pack Objects Limiting: [Feature flag] Enable pack objects limiting for... (gitlab-org/gitaly#4413 - closed).

Playgrounds:

• Gitaly: Overview: https://dashboards.gitlab.net/dashboard/snapshot/BoR4ey27JgnK1Bwx1qOJhN8hlQAnq0zo?orgId=1

Screenshot

• Gitaly: Host detail: https://dashboards.gitlab.net/dashboard/snapshot/jTN0ujIK9uij0CQpa397AHNhsvWKd1xI?orgId=1. The last two dashboards are empty because we haven't enabled Pack Object Limiting, yet.

Screenshot