Add option to track non-namespaces Sidekiq keys

With the probe_namespaced and probe_non_namespaced options, we can track both namespaced and non-namespaced keys during namespace deprecation.

See gitlab-com/gl-infra/scalability#2285

Local verification

1. spin up a Redis instance redis-server and update config/redis.queues.yml in your GDK's gitlab folder path to:

➜  gitlab git:(sc1-skfetcher-dual-fetch) ✗ cat config/redis.queues.yml
---
development: redis://localhost:6379

2. Start a metric server via https://gitlab.com/gitlab-org/gitlab-exporter#running-gitlab-exporter-as-a-web-exporter. Run the curl command to fetch sidekiq metrics

curl localhost:9168/sidekiq

3. The actual namespaced vs non-namespaced workloads can be generated using this MR gitlab-org/gitlab!116379 (merged).