Tune workflow and job duration histograms for CI workloads

[darthfork]

Parent issue: #34
Parent issue: #36

Summary

Workflow and job duration histograms currently use Prometheus default buckets, which are not well matched to CI durations that often span seconds to hours.

Goals

• Tune bucket boundaries for CI jobs and workflows.
• Ensure bucket count and distribution balance query value against storage cost.
• Consider whether separate workflow and job bucket configs should be supported.

Acceptance criteria

• Histogram buckets reflect realistic CI timing ranges.
• Bucket choices are documented and justified.