Instrument cache fetch hit and misses

Paco Guzmanrequested to merge
instrument-cache-fetch-hit-and-misses into master

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?

Merge request reports