Add new Redis singleton to shard out feature flags

What does this MR do and why?

This MR adds Gitlab::Redis::FeatureFlag to shard out feature flag workloads into its own Redis instance.

No MultiStore is used since MultiStore uses feature flags internally which will create a recursive loop (MultiStore calls Feature, whose l2 cache is a cache store using a MultiStore etc etc). Instead, the rollout/cutover will be done on https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-com and a change management issue will be needed.

See gitlab-com/gl-infra/scalability#2335 (comment 1378844444) for background.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Prepare 2 shells - one for gdk rails c and another for gdk redis-cli -n <db number>

Part 1: FeatureFlag singleton connects to cache db if no feature-flag db is defined

1. Check your config/redis.cache.yml and ensure config/redis.yml is empty and config/redis.feature_flag.yml does not exist.

➜  gitlab git:(sc1-shard-feature-flag) cat config/redis.cache.yml
---
development: unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=2
test: unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=12

2. Get into your redis-cli with gdk redis-cli -n 2
3. Enable a random feature flag on Rails console (e.g. webauthn_without_totp). 1st command looks up pg while 2nd command hits the Redis cache.

[1] pry(main)> Feature.enabled?(:webauthn_without_totp)
  Feature::FlipperGate Load (1.2ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'webauthn_without_totp' /*application:console,db_config_name:main,console_hostname:Sylvesters-MacBook-Pro-2.local,console_username:sylvesterchin,line:/lib/feature.rb:237:in `block in current_feature_value'*/
=> false
[2] pry(main)> Feature.enabled?(:webauthn_without_totp)
=> false

4. Check redis-cli to see key is present

redis /Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket[2]> keys *webauthn_without_totp*
1) "cache:gitlab:flipper/v1/feature/webauthn_without_totp"

Part 2: Updating config/redis.yml

1. Update config/redis.yml

➜  gitlab git:(sc1-shard-feature-flag) cat config/redis.yml
---

development:
  feature_flag:
    url: unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=3

2. Ensure current Rails console is connected to cache db

[2] pry(main)> Gitlab::Redis::FeatureFlag.url
=> "unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=2"

3. Restart Rails console
4. Check that Gitlab::Redis::FeatureFlag is connected to the feature flag db

[2] pry(main)> Gitlab::Redis::FeatureFlag.url
=> "unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=3"

5. Check that db 3 does not have the feature flag key

redis /Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket[3]> keys *webauthn_without_totp*
(empty array)

6. Run feature flag check command (notice that a look-up to pg is performed due to cache miss)

[1] pry(main)> Feature.enabled?(:webauthn_without_totp)
  Feature::FlipperGate Load (0.6ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'webauthn_without_totp' /*application:console,db_config_name:main,console_hostname:Sylvesters-MacBook-Pro-2.local,console_username:sylvesterchin,line:/lib/feature.rb:237:in `block in current_feature_value'*/
=> false

7. Check that the feature flag db now contains the feature flag key

redis /Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket[3]> keys *webauthn_without_totp*
1) "cache:gitlab:flipper/v1/feature/webauthn_without_totp"

MR acceptance checklist

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