feat(metrics): Implement exemplar metrics - #239
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Add owned fixed-bucket storage, direct protobuf encoding, and support for the legacy HistogramBuilder API (only classic histograms in this commit).
Choose to pre-sort histogram buckets over a sorted boolean variable. Also satisfies cargo clippy & cargo fmt
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…omplete, non-empty names
… names Legacy metric and label names must match Prometheus' name grammars. Names outside these grammars cause Prometheus to reject the scrape, and the grammars exclude non-ASCII characters. Foundations now quotes label names and uses OpenMetrics' quoted metric-name syntax when required. This allows UTF-8 and other nonstandard names without affecting legacy compatibility.
Add registerable counter, classic histogram, and native histogram types that retain exemplars alongside observations. Support typed and legacy label sets, Family usage, OpenMetrics text, and Prometheus protobuf encoding. Validate exemplars during collection and report serialisation failures as non-fatal diagnostics.
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Overview
Adds first-party exemplar-aware metric types for counters, classic histograms, and native histograms while leaving existing metric types unchanged.
Public API
CounterWithExemplarHistogramWithExemplarsNativeHistogramWithExemplarsExemplaraccessors for labels and sampled values are exposed throughCounterWithExemplar::get.These types integrate with
Family, registration, collection, and the existing protobuf and OpenMetrics encoders.Recording Semantics
Recording does not require exemplar labels to implement
Serialize; serialisation occurs when the metric is encoded.Encoding
Counter and classic-histogram exemplars are emitted in both protobuf and OpenMetrics text.
Native-histogram exemplars:
Empty counter and classic-histogram exemplar label sets remain valid and are emitted in text as {}.
Labels And Validation
Exemplar labels use the existing label serialiser and support:
Invalid, duplicate, or unserialisable exemplar labels drop only the exemplar. The metric sample remains available, and a non-fatal diagnostic is emitted.
Native-histogram exemplars without the required timestamp are removed with a non-fatal diagnostic; the native histogram itself is still emitted.