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## Problem We lack insight into: - How much of a tenant's physical size is image vs. delta layers - Average sizes of image vs. delta layers - Total layer counts per timeline, indicating size of index_part object As well as general observability love, this is motivated by https://github.com/neondatabase/neon/issues/6738, where we need to define some sensible thresholds for storage amplification, and using total physical size may not work well (if someone does a lot of DROPs then it's legitimate for the physical-synthetic ratio to be huge), but the ratio between image layer size and delta layer size may be a better indicator of whether we're generating unreasonable quantities of image layers. ## Summary of changes - Add pageserver_layer_bytes and pageserver_layer_count metrics, labelled by timeline and `kind` (delta or image) - Add & subtract these with LayerInner's lifetime. I'm intentionally avoiding using a generic metric RAII guard object, to avoid bloating LayerInner: it already has all the information it needs to update metric on new+drop.