Instrument cache fetch hit and misses

What does this MR do?

Add metrics to cache hits and cache misses

Why was this MR needed?

To have a better knowledge in possible improvements on cache strategies

What are the relevant issue numbers?

It's related with https://gitlab.com/gitlab-org/gitlab-ce/issues/18709 and https://gitlab.com/gitlab-com/performance/issues/10

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together