Add reset function to Histogram#61
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Adds a `reset` to `System.Metrics.Prometheus.Metric.Histogram` that zeroes the bucket counts, sum, and observation count while leaving the bucket upper bounds unchanged. This mirrors `set 0` on `Counter`/`Gauge` and is useful for resetting metrics between tests when they are defined as top-level values. The reset uses `atomicModifyIORef'` for consistency with `observe`, so it is safe against concurrent observations. Closes #55 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
brandonchinn178
approved these changes
Jun 29, 2026
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Adds a
resetfunction toSystem.Metrics.Prometheus.Metric.Histogram, resolving #55.CounterandGaugecan already be reset withset 0, butHistogramhad no equivalent.resetzeroes the bucket counts, sum, and observation count while leaving the bucket upper bounds unchanged, putting the histogram back into the same state as a freshly-created one. The primary use case is resetting metrics between tests when they're defined as top-level values.Notes
atomicModifyIORef'(matchingobserve/observeAndSample) so it's safe against concurrent observations — the prototype in the issue used the non-atomicmodifyIORef'.lekeys, including+Inf) are preserved; only the accumulated values are zeroed, which keeps it within the Prometheus client-spec rule that buckets must not change after creation.resetis not part of the Prometheus client library spec (which only mandatesobservefor histograms). On a live scraped metric a reset reads as a counter reset / process restart to Prometheus, whichrate/increasehandle, but it's intended for test isolation. The Haddock spells this out.Test plan
reset, thensample, and confirm all bucket counts,histSum, andhistCountare 0 while the bucket keys are unchanged.🤖 Generated with Claude Code