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## Problem Compaction jobs and other background loops are concurrency-limited through a global semaphore. The current counters allow quantifying how _many_ tasks are waiting. But there is no way to tell how _much_ delay is added by the semaphore. So, add a counter that aggregates the wall clock time seconds spent acquiring the semaphore. The metrics can be used as follows: * retroactively calculate average acquisition time in a given time range * compare the degree of background loop backlog among pageservers The metric is insufficient to calculate * run-up of ongoing acquisitions that haven't finished acquiring yet * Not easily feasible because ["Cancelling a call to acquire makes you lose your place in the queue"](https://docs.rs/tokio/latest/tokio/sync/struct.Semaphore.html#method.acquire) ## Summary of changes * Refactor the metrics to follow the current best practice for typed metrics in `metrics.rs`. * Add the new counter.