Add Prometheus histogram for Puma readiness probe duration

Stan Hurequested to merge
sh-add-prom-metrics-workhorse-puma-readiness into master

What does this MR do and why?

Add workhorse_puma_readiness_probe_duration_seconds as a HistogramVec with a result label ("ok"/"error") to measure HTTP response times for the Puma /-/readiness endpoint. This enables p50/p95/p99 latency analysis to evaluate the effectiveness of Workhorse load shedding.

Uses default Prometheus histogram buckets for consistency with the existing workhorse_health_check_duration_seconds metric.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

References

Relates to gitlab-com/gl-infra/production#21451 (closed)

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Edited by Stan Hu

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