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om2: MetricFamilyName is MetricName #2791

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35 changes: 21 additions & 14 deletions docs/specs/om/open_metrics_spec_2_0.md
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Original file line number Diff line number Diff line change
Expand Up @@ -87,7 +87,7 @@ CompositeValue MUST contain all information necessary to recreate a sample value
The following Metric Types MUST use CompositeValue for Metric Values:

* [Histogram](#histogram) MetricFamily Type.
* [GaugeHistogram](#gauge-histogram) MetricFamily Type.
* [GaugeHistogram](#gaugehistogram) MetricFamily Type.
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@bwplotka bwplotka Feb 10, 2026

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FYI: Broken link

* [Summary](#summary) MetricFamily Type.

Other Metric Types MUST use Numbers.
Expand Down Expand Up @@ -146,27 +146,34 @@ A MetricFamily MAY have zero or more Metrics. A MetricFamily MUST have a name, H

##### Name

MetricFamily names are a string and MUST be unique within a MetricSet. Names SHOULD be in snake_case. Names SHOULD follow the restrictions in the ABNF section under `metricname`. Metric names MAY be any quoted and escaped UTF-8 string as described in the ABNF section. Be aware that exposing UTF-8 metrics is still experimental and may reduce usability, especially when suffixes are not included.
MetricFamily name:

* MUST be string.
* MUST be unique within a MetricSet.
* MUST be the same as every MetricPoint's MetricName in the family.

> NOTE: [OpenMetrics 1.0](https://prometheus.io/docs/specs/om/open_metrics_spec/#suffixes) required mandatory suffixes
> for MetricName and matching MetricFamily names without such suffixes. To improve parser reliability (i.e. matching
> [MetricFamily metadata](#metricfamily-metadata)) and future compatibility, this specification requires MetricFamily name to strictly match MetricNames
> in the same family.

Names SHOULD be in snake_case. Names SHOULD follow the restrictions in the ABNF section under `metricname`. Metric names MAY be any quoted and escaped UTF-8 string as described in the ABNF section. Be aware that exposing UTF-8 metrics may reduce usability, especially when `_total` or unit suffixes are not included in the names.

Colons in MetricFamily names are RESERVED to signal that the MetricFamily is the result of a calculation or aggregation of a general purpose monitoring system.
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@ArthurSens ArthurSens Dec 3, 2025

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I don't know if this holds any value in the spec. When we mention 'RESERVED', are we instructing SDKs not to allow colons?

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@dashpole dashpole Dec 3, 2025

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I also don't know what RESERVED means? I don't think it is one of the keywords (like MUST, SHOULD, MUST NOT).

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@dashpole dashpole Dec 3, 2025

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I think we can say "MUST NOT be used unless ..."

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@bwplotka bwplotka Feb 10, 2026

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I will keep RESERVED as it was there before my change. We use RESERVED word 6 times already in this document, so let's have a dedicated PR to change it.

I think it's fine, but to be strict with https://datatracker.ietf.org/doc/html/rfc2119 we could check MUST NOT.

Let's focus on suffixes in this PR, WDYT?

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@dashpole dashpole Feb 10, 2026

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thats also fine by me


MetricFamily names beginning with underscores are RESERVED and MUST NOT be used unless specified by this standard.

###### Suffixes
###### Reserved Suffixes

The name of a MetricFamily MUST NOT result in a potential clash for sample metric names as per the ABNF with another MetricFamily in the Text Format within a MetricSet. An example would be a gauge called "foo_total" as a counter called "foo" could create a "foo_total" in the text format.
MetricFamily name SHOULD NOT end with `_count`, `_sum`, `_gcount`, `_gsum`, `_bucket`. Specifically name MUST NOT create
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@ywwg ywwg Feb 10, 2026

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Suggested change
MetricFamily name SHOULD NOT end with `_count`, `_sum`, `_gcount`, `_gsum`, `_bucket`. Specifically name MUST NOT create
MetricFamily name SHOULD NOT end with `_count`, `_sum`, `_gcount`, `_gsum`, `_bucket`. Specifically a name MUST NOT create a MetricName collision when converted to [the Text OpenMetrics 1.0](https://prometheus.io/docs/specs/om/open_metrics_spec).

MetricName collision when converted to [the Text OpenMetrics 1.0](https://prometheus.io/docs/specs/om/open_metrics_spec).
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@ywwg ywwg Feb 10, 2026

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nit - are we manually word-wrapping or no? Other paragraphs seem not to have newlines mid-paragraph. The markdown format doesn't care but we might as well be consistent.


Exposers SHOULD avoid names that could be confused with the suffixes that text format sample metric names use.
A non-compliant example would be a gauge called `foo_bucket` and a histogram called `foo`. Exposers negotiating the older
OpenMetrics or Text formats, or ingestors which support only the older data model could end up storing `foo` histogram
in a classic representation (`foo_bucket`, `foo_count`, `foo_sum`) causing clash with the gauge and scrape rejection or dropped data.
Comment on lines +171 to +173
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@ywwg ywwg Feb 10, 2026

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Suggested change
A non-compliant example would be a gauge called `foo_bucket` and a histogram called `foo`. Exposers negotiating the older
OpenMetrics or Text formats, or ingestors which support only the older data model could end up storing `foo` histogram
in a classic representation (`foo_bucket`, `foo_count`, `foo_sum`) causing clash with the gauge and scrape rejection or dropped data.
A non-compliant example would be a gauge called `foo_bucket` and a histogram called `foo`. Exposers negotiating the older OpenMetrics or Text formats, or ingestors which support only the older data model could end up storing the `foo` histogram in the classic representation (`foo_bucket`, `foo_count`, `foo_sum`), which would clash with the gauge and cause a scrape rejection or dropped data.


* Suffixes for the respective types are:
* Counter: `_total`
* Summary: `_count`, `_sum`, `` (empty)
* Histogram: `_count`, `_sum`, `_bucket`
* GaugeHistogram: `_gcount`, `_gsum`, `_bucket`
* Info: `_info`
* Gauge: `` (empty)
* StateSet: `` (empty)
* Unknown: `` (empty)
> This rule is because this specification is following a shift in Prometheus ecosystem towards [composite values](#compositevalues) instead of [the "classic" representation](https://prometheus.io/docs/specs/om/open_metrics_spec/#histogram-1). However, this transformation will take time.
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@ywwg ywwg Feb 10, 2026

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Suggested change
> This rule is because this specification is following a shift in Prometheus ecosystem towards [composite values](#compositevalues) instead of [the "classic" representation](https://prometheus.io/docs/specs/om/open_metrics_spec/#histogram-1). However, this transformation will take time.
> This rule exists because this specification is following a shift in Prometheus ecosystem towards [composite values](#compositevalues) instead of [the "classic" representation](https://prometheus.io/docs/specs/om/open_metrics_spec/#histogram-1). However, this transformation will take time.

> Reserving suffixes improves compatibility with older ingestors and the eventual migration process.

##### Type

Expand Down