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OM2: Remove exemplar hard limit; mention use intention #2835

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bwplotka merged 2 commits into from
Feb 11, 2026
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OM2: Remove exemplar hard limit; mention use intention #2835

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OM2: Remove exemplar hard limit; mention use intention
bwplotka Feb 10, 2026
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6 changes: 2 additions & 4 deletions docs/specs/om/open_metrics_spec_2_0.md
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Expand Up @@ -128,9 +128,9 @@ Exemplars are references to data outside of the MetricSet. A common use case are

Exemplars MUST consist of a LabelSet and a value, and SHOULD have a timestamp. They MAY each be different from the MetricPoints' LabelSet and timestamp.

The combined length of the label names and values of an Exemplar's LabelSet MUST NOT exceed 128 UTF-8 character code points. Other characters in the text rendering of an exemplar such as `",=` are not included in this limit for implementation simplicity and for consistency between the text and proto formats.
While there's no [hard limit](#size-limits) specified, Exemplar's LabelSet SHOULD NOT be used to transport large data like tracing span details or other event logging.

Ingestors MAY discard exemplars.
Ingestors MAY truncate the Exemplar's LabelSet or discard Exemplars.

#### Metric

Expand Down Expand Up @@ -1438,8 +1438,6 @@ The number of unique time series is roughly equivalent to the number of non-comm

If all targets of a particular type are exposing the same set of time series, then each additional targets' strings poses no incremental cost to most reasonably modern monitoring systems. If however each target has unique strings, there is such a cost. As an extreme example, a single 10k character metric name used by many targets is on its own very unlikely to be a problem in practice. To the contrary, a thousand targets each exposing a unique 36 character UUID is over three times as expensive as that single 10k character metric name in terms of strings to be stored assuming modern approaches. In addition, if these strings change over time older strings will still need to be stored for at least some time, incurring extra cost. Assuming the 10 million times series from the last paragraph, 100MB of unique strings per hour might indicate a use case for then the use case may be more like event logging, not metric time series.

There is a hard 128 UTF-8 character limit on exemplar length, to prevent misuse of the feature for tracing span data and other event logging.

## Security

Implementors MAY choose to offer authentication, authorization, and accounting; if they so choose, this SHOULD be handled outside of OpenMetrics.
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