Add metrics for Redis Cluster redirection

What does this MR do and why?

This MR adds a new counter gitlab_redis_client_redirections_total which tracks the number of MOVED and ASK errors. Such redirection errors are retried in https://github.com/redis/redis-rb/blob/v4.8.0/lib/redis/cluster.rb#L220 which usually gets redirected to the correct node. During cluster re-sharding or possibly improper client setup (see issue below), it would lead to a false positive in the service error ratio.

See related issue: gitlab-com/gl-infra/scalability#2212 (closed)

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

Setup: Follow step 1-4 in gitlab-com/gl-infra/scalability#2212 (comment 1287797884)

Option 1: DIY

1. Open GDK and trigger some MOVED or ASK

Gitlab::Redis::ClusterRateLimiting.with{|c| c.get('b')} # in this case slot 3300 was being fiddled with so `b` would give us problems

Option 2: Run the specs in this MR

Verify via gdk (check if metrics are enabled)

[10] pry(main)> Gitlab::Metrics.metrics_folder_present?
=> true
[10] pry(main)> require 'prometheus/client/formats/text.rb'
=> true
[14] pry(main)> puts Prometheus::Client::Formats::Text.marshal_multiprocess.split("\n").filter{|x| x.include?('gitlab_redis_client_redirections_total')}
# HELP gitlab_redis_client_redirections_total Multiprocess metric
# TYPE gitlab_redis_client_redirections_total counter
gitlab_redis_client_redirections_total{node_key="127.0.0.1:6380",redirection_type="ASK",slot="123",storage="shared_state"} 4
gitlab_redis_client_redirections_total{node_key="127.0.0.1:6380",redirection_type="MOVED",slot="123",storage="shared_state"} 5
gitlab_redis_client_redirections_total{node_key="127.0.0.1:7001",redirection_type="ASK",slot="3300",storage="cluster_rate_limiting"} 1
=> nil

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.