Record enqueuing rate for Sidekiq jobs

What does this MR do?

It introduces a Gitlab::SidekiqMiddleware::ClientMetrics middleware which is used to write a new Counter metric for enqueued jobs (sidekiq_enqueued_jobs_total). It reuses the worker labels from the ServerMetrics (previously Metrics) class, which includes the queue and further worker metadata.

Example of the record:

sidekiq_enqueued_jobs_total{queue="project_export",latency_sensitive="no",external_dependencies="no",feature_category="",boundary=""} 1

gitlab-com/gl-infra/scalability#91 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content