Function read_latency_gauges 

fn read_latency_gauges(registry: &MetricsRegistry)

Registers the file store’s read-latency histogram as one computed gauge per bucket, since the registry has no computed histogram. Each gauge carries an le label, the bucket’s upper bound in seconds, and counts the reads under that bound, so the series aggregate like the buckets of a Prometheus histogram. Bucket i below the last holds reads under 32 µs << i, and the last bucket is +Inf.