The core engine histograms ship with default bucket boundaries tuned for typical serving workloads. The `--custom-histogram-buckets` option replaces the boundaries of one or more bucket families. Use it, for example, to track sub-300ms latency SLOs with the request-phase histograms, whose smallest default boundary is 0.3s. Signed-off-by: Guy Stone <guys@spotify.com> Co-authored-by: Claude Fable 5 <noreply@anthropic.com>