Track duration spent in concurrency limit queue

What does this MR do and why?

This MR updates .concurrency_limit_resume to set the timestamp when the job is added to the concurrency limit queue. We use the term "buffered" to minimise confusion between various queues. "deferred" is used for the skip jobs middleware.

This updates the scheduling latency to track the duration starting from when the job is added to the concurrency limit queue.

See gitlab-com/gl-infra/scalability#3877

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