Extract metadata object for Redis cache

What does this MR do and why?

Contributes to #383458 (closed)

Refactoring for Gitlab::Cache::Metrics class. I extracted a metadata object that is going to be used for Gitlab::Cache::Client class.

It is a preparation part for Gitlab::Cache::Client implementation.

Roadmap

Step	MR
Refactor Gitlab::Cache::Metrics	👈 this MR
Create Gitlab::Cache::Client	!111210 (merged)

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