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## Problem The current global `pageserver_layers_visited_per_vectored_read_global` metric does not appear to accurately measure read amplification. It divides the layer count by the number of reads in a batch, but this means that e.g. 10 reads with 100 L0 layers will only measure a read amp of 10 per read, while the actual read amp was 100. While the cost of layer visits are amortized across the batch, and some layers may not intersect with a given key, each visited layer contributes directly to the observed latency for every read in the batch, which is what we care about. Touches https://github.com/neondatabase/cloud/issues/23283. Extracted from #10566. ## Summary of changes * Count the number of layers visited towards each read in the batch, instead of the average across the batch. * Rename `pageserver_layers_visited_per_vectored_read_global` to `pageserver_layers_per_read_global`. * Reduce the read amp log warning threshold down from 512 to 100.