Add instrumentation for cache efficiency metrics

What does this MR do and why?

Problem

We don't have a common library to calculate hit ratio and cache generation duration metrics.

Solution

Add a class Gitlab::Cache::Metrics with methods to support hit-ratio and cache generation duration metrics.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.