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## Problem The `evictions_with_low_residence_duration` is used as an indicator of cache thrashing. However, there are situations where it is quite legitimate to only have a short residence during compaction, where a delta is downloaded, used to generate an image layer, and then discarded. This can lead to false positive alerts. ## Summary of changes - Only track low residence duration for layers that have been accessed at least once (compaction doesn't count as an access). This will give us a metric that indicates thrashing on layers that the _user_ is using, rather than those we're downloading for housekeeping purposes. Once we add "layer visibility" as an explicit property of layers, this can also be used as a cleaner condition (residence of non-visible layers should never be alertable)