Draft: Add new redis instance

What does this MR do and why?

Part of gitlab-com/gl-infra/scalability#1820 (closed)

How to set up and validate locally

Using the default fallback to Cache

Start the rails console and try the following:

[1] pry(main)> Gitlab::Redis::RepositoryCache.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::RepositoryCache, :path=>"/home/bob/gitlab-development-kit/redis/redis.socket"}
[2] pry(main)> Gitlab::Redis::Cache.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::Cache, :path=>"/home/bob/gitlab-development-kit/redis/redis.socket"}
[3] pry(main)> Gitlab::Redis::Cache.with { |r| r.set('hello', 'world') }
=> "OK"
[4] pry(main)> Gitlab::Redis::RepositoryCache.with { |r| r.get('hello') }
=> "world"
[5] pry(main)> Gitlab::Redis::RepositoryCache.with { |r| r.set('hello', 'mars') }
=> "OK"
[6] pry(main)> Gitlab::Redis::Cache.with { |r| r.get('hello') }
=> "mars"
[7] pry(main)> Gitlab::Redis::RepositoryCache.with { |r| r.get('hello') }
=> "mars"

Notice that both instances use the same database.

Using a separate RepositoryCache configuration

Create a config/redis.repository_cache.yml file with the following contents:

development:
  url: unix:/home/bob/gitlab-development-kit/redis/redis.socket?db=10
test:
  url: unix:/home/bob/gitlab-development-kit/redis/redis.socket?db=100

Restart the rails console. Try the following in a rails console: notice how the 2 databases are split:

[1] pry(main)> Gitlab::Redis::RepositoryCache.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::RepositoryCache, :path=>"/home/bob/gitlab-development-kit/redis/redis.socket", :db=>10}
[2] pry(main)> Gitlab::Redis::Cache.params                                                                                                                 
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::Cache, :path=>"/home/bob/gitlab-development-kit/redis/redis.socket"}
[3] pry(main)> Gitlab::Redis::Cache.with { |r| r.set('hello', 'world') }
=> "OK"
[5] pry(main)> Gitlab::Redis::RepositoryCache.with { |r| r.get('hello') }                                                                                  
=> nil
[6] pry(main)> Gitlab::Redis::RepositoryCache.with { |r| r.set('hello', 'mars') }                                                                          
=> "OK"
[7] pry(main)> Gitlab::Redis::Cache.with { |r| r.get('hello') }                                                                                            
=> "world"
[8] pry(main)> Gitlab::Redis::RepositoryCache.with { |r| r.get('hello') }                                                                                  
=> "mars"

MR acceptance checklist

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