Add configurable summary quantile metrics via --metrics.mode flag

[Alb0t]

Summary

• Adds --metrics.mode flag to control latency metric type: histogram (default, unchanged behavior), summary (quantiles only), or both (histogram + summary side by side)
• Adds --quantiles flag to specify which quantiles to track (default: 0.5,0.9,0.95,0.99)
• In both mode, histogram keeps existing name smokeping_response_duration_seconds, summary uses smokeping_response_duration_summary_seconds to avoid _sum/_count collision
• In summary-only mode, summary uses smokeping_response_duration_seconds for backward compatibility
• Epsilon (accuracy) values are auto-assigned based on quantile position — not exposed to operators
• Fully backward compatible: default behavior unchanged, no new metrics unless opted in
• Config reload (SIGHUP / /-/reload) respects mode and quantile settings

Usage

Default — histogram only (unchanged)

smokeping_prober host1 host2

Both histogram buckets + summary quantiles

smokeping_prober --metrics.mode both host1 host2

Summary quantiles only, custom set

smokeping_prober --metrics.mode summary --quantiles "0.5,0.99" host1 host2