Auto merge of #4121 - Turbo87:background-job-metrics, r=pietroalbini
Auto merge of #4121 - Turbo87:background-job-metrics, r=pietroalbini

metrics/service: Add `background_jobs` metric

This can be used to monitor how long the queue of background jobs currently is.

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io