Add Redis instrumentation before `ensure_connected`

What does this MR do and why?

This MR adds Redis instrumentation before ensure_connected. This allows us to check for connection-based errors like EOFError and EPIPE before they are rescued and retried in ensure_connected.

We introduce gitlab_redis_client_connection_exceptions_total to avoid creating noise to the various Redis's SLI error rates.

See discussion in gitlab-com/gl-infra/scalability#2564 (comment 1665334335)

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Before	After

How to set up and validate locally

1. Open a gdk rails console. Start by running:

require 'prometheus/client/formats/text.rb'
rc = Gitlab::Redis::Queues.with {|c| c }
rc.ping # "PONG"

2. Set redis timeout to 5s

gdk redis-cli -n 1 config set timeout 5

3. Run another rc.ping and check that the stats changed (increased by 1).

[6] pry(main)> Prometheus::Client::Formats::Text.marshal_multiprocess.split("\n").filter{|x| x.include?("gitlab_redis_client_connection_exceptions_total")}
=> ["# HELP gitlab_redis_client_connection_exceptions_total Multiprocess metric",
 "# TYPE gitlab_redis_client_connection_exceptions_total counter",
 "gitlab_redis_client_connection_exceptions_total{exception=\"Redis::ConnectionError\",storage=\"queues\"} 2",
[7] pry(main)> rc.ping
=> "PONG"
[8] pry(main)> Prometheus::Client::Formats::Text.marshal_multiprocess.split("\n").filter{|x| x.include?("gitlab_redis_client_connection_exceptions_total")}
=> ["# HELP gitlab_redis_client_connection_exceptions_total Multiprocess metric",
 "# TYPE gitlab_redis_client_connection_exceptions_total counter",
 "gitlab_redis_client_connection_exceptions_total{exception=\"Redis::ConnectionError\",storage=\"queues\"} 3",

MR acceptance checklist

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

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