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Allow multiple exemplars on counters and histograms #2838

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Allow multiple exemplars on counters and histograms #2838

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4 changes: 2 additions & 2 deletions docs/specs/om/open_metrics_spec_2_0.md
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Expand Up @@ -214,7 +214,7 @@ A MetricPoint in a Metric with the type Counter SHOULD have a Timestamp value ca

A MetricPoint in a Metric's Counter's Total MAY reset to 0. If present, the corresponding Start Timestamp MUST also be set to the timestamp of the reset.

A MetricPoint in a Metric's Counter's Total MAY have an exemplar.
A MetricPoint in a Metric's Counter's Total MAY have exemplars.

#### StateSet

Expand Down Expand Up @@ -286,7 +286,7 @@ Exposed Classic Bucket thresholds SHOULD stay constant over time and between tar

If the NaN value is allowed, it MUST be counted in the +Inf bucket, and MUST NOT be counted in any other bucket. The rationale is that NaN does not belong to any bucket mathematically, however instrumentation libraries traditionally put it into the +Inf bucket.

Classic Bucket values MAY have exemplars. The value of the exemplar MUST be within the Classic Bucket. Exemplars SHOULD be put into the Classic Bucket with the lowest threshold that includes the exemplar value. A Classic Bucket MUST NOT have more than one exemplar.
Classic Bucket values MAY have exemplars. The value of the exemplar SHOULD be within the Classic Bucket. Exemplars SHOULD be put into the Classic Bucket with the lowest threshold that includes the exemplar value. A Classic Bucket SHOULD NOT have more than one exemplar.
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@bwplotka bwplotka Feb 11, 2026

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WG notes

  • It's typical in Otel to go outside
  • How this works for native histograms? Separate storages? We could switch to the language native histograms uses - "evenly distributed". As long as it's evenly distributed, it's fine if it's not within buckets.
  • Krajo: For classic histograms are by design it's inside bucket. For classic histograms we can have only one exemplar per bucket. We can refine this to say similar thing to native histograms. In Go SDK exemplars storages are different for native vs classic (although on proto it's one field - to be checked).

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@bwplotka bwplotka Feb 11, 2026

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  • How this will work with 1.0 compatible storage

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@bwplotka bwplotka Feb 11, 2026

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  • K: Maybe we should say one exemplar per bucket? We could change over time.
  • D: What's value of saying MUST
  • K: Either be strict (same as before), or be relaxed and lose exemplars for 1.0 - it's likely fine

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K: We need to rewrite this rule as this applies in a different way (composite) - should be one per bucket / align wording


##### Native Buckets

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