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Add metric exemplar support - #67
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Instruments now record exemplars (a span, its measurement value, and the aggregation attributes) via addExemplar(span:value:attributes:), stored thread-safely alongside their values and drained on snapshotAndReset. Tracer+Metrics records an exemplar in the retire callback where the real measurement value is known. The exporter attaches exemplars to the matching data point (by aggregation key) and, via Exporter.exemplarSamplingDecision, lets a reporter gate attachment on the trace sampling decision. ExampleReporter demonstrates reusing its per-span decision for exemplars. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Add a DurationUnit enum (seconds, milliseconds, microseconds, nanoseconds) to reportAsDurationHistogramMetric, defaulting to milliseconds. The unit drives both the metric's UCUM symbol (s/ms/µs/ns) and the conversion applied to each recorded span duration. Collapse the four Duration conversion properties into a single wholeUnits helper that derives the half-unit rounding threshold from the per-unit amount, and add asMicroseconds/asSeconds. Also rename the exemplar helper's parameter key -> metricAttributes, and make the duration histogram tests deterministic by asserting against the span's own frozen elapsed (the value is recorded during end(), so wall-clock time can't be cancelled after the fact). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
ladd
marked this pull request as ready for review
July 17, 2026 19:38
jparise
approved these changes
Jul 17, 2026
Co-authored-by: Jon Parise <jon.parise@airbnb.com>
bachand
reviewed
Jul 20, 2026
| timeUnixNano: timeUnixNano, | ||
| asDouble: doubleValue, | ||
| asInt: intValueString, | ||
| exemplars: nil, // no exemplar support yet |
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| // Decision drops everything: no exemplars are attached. | ||
| let noneExporter = Exporter(timeReference: timeReference, exemplarSamplingDecision: { _ in false }) | ||
| #expect(noneExporter.convertToOTLP(exemplars: counter.exemplars, metricAttributes: [:]) == nil) | ||
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| // Decision keeps only the sampled span. | ||
| let selectiveExporter = Exporter(timeReference: timeReference, exemplarSamplingDecision: { $0.id == sampledSpan.id }) | ||
| let attached = try #require(selectiveExporter.convertToOTLP(exemplars: counter.exemplars, metricAttributes: [:])) | ||
| #expect(attached.map(\.spanId) == [sampledSpan.id]) | ||
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| // Default decision attaches every exemplar. | ||
| let defaultExporter = Exporter(timeReference: timeReference) | ||
| #expect(defaultExporter.convertToOTLP(exemplars: counter.exemplars, metricAttributes: [:])?.count == 2) |
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| // Exemplars move to the snapshot; the live instrument is reset. | ||
| #expect(snapshot.exemplarSpans.map(\.id) == [span.id]) | ||
| #expect(counter.exemplarSpans.isEmpty) |
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| /// Reporters typically drive this from the current trace sampling decision, so that exemplars — which | ||
| /// link a metric data point back to a trace — are only attached when that trace is sampled. |
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Summary
Adds OpenTelemetry metric exemplar support — a sampled measurement plus a link back to the trace that produced it.
addExemplar(span:value:attributes:), stored thread-safely (Mutex<[Exemplar<T>]>) alongside their values and drained onsnapshotAndReset().Tracer+Metricsrecords the exemplar inside the span retire callback, where the real measurement value (and collected attributes) is known.asDouble/asInt, span/trace IDs, andfilteredAttributes.Exporter.exemplarSamplingDecisionlets a reporter gate exemplar attachment on the trace sampling decision.ExampleReporterdemonstrates reusing its per-span decision.parentIdpublic for ease of integration testing.addExemplarstays on the concrete generic instruments (likeadd/record/observe);exemplarSpansremains on theInstrumentprotocol for type-erased read access.Testing
swift test— all suites pass, including new coverage for exemplar wiring, attribute matching, snapshot/reset, and the sampling decision.@jparise
@bachand