feat: implement histogram and gauge histogram support for OpenMetrics 2.0

[dashpole]

Part 2 of #893

Implement serialization and strict validation for classic, native, dual, and gauge histograms in OpenMetrics 2.0 format according to the specification and ABNF grammar.

goos: linux
goarch: amd64
pkg: github.com/prometheus/common/expfmt
cpu: AMD EPYC 7B12
                                             │    sec/op     │     B/op      │   allocs/op   │
ConvertMetricFamily/TEXT/COUNTER-24              502.8n ± 2%      0.000 ± 0%     0.000 ± 0%
ConvertMetricFamily/OM1.0/COUNTER-24             1.211µ ± 4%      33.00 ± 0%     2.000 ± 0%
ConvertMetricFamily/OM2.0/COUNTER-24             1.100µ ± 5%      0.000 ± 0%     0.000 ± 0%
ConvertMetricFamily/TEXT/GAUGE-24                487.5n ± 5%      0.000 ± 0%     0.000 ± 0%
ConvertMetricFamily/OM1.0/GAUGE-24               735.8n ± 5%      1.000 ± 0%     1.000 ± 0%
ConvertMetricFamily/OM2.0/GAUGE-24               653.2n ± 3%      0.000 ± 0%     0.000 ± 0%
ConvertMetricFamily/TEXT/UNTYPED-24              428.9n ± 4%      0.000 ± 0%     0.000 ± 0%
ConvertMetricFamily/OM1.0/UNTYPED-24             599.5n ± 8%      1.000 ± 0%     1.000 ± 0%
ConvertMetricFamily/OM2.0/UNTYPED-24             573.3n ± 7%      0.000 ± 0%     0.000 ± 0%
ConvertMetricFamily/TEXT/SUMMARY-24              2.125µ ± 4%      56.00 ± 0%     2.000 ± 0%
ConvertMetricFamily/OM1.0/SUMMARY-24             2.745µ ± 7%      57.00 ± 0%     3.000 ± 0%
ConvertMetricFamily/TEXT/HISTOGRAM-24            4.298µ ± 7%      288.0 ± 0%     6.000 ± 0%
ConvertMetricFamily/OM1.0/HISTOGRAM-24           5.265µ ± 5%      289.0 ± 0%     7.000 ± 0%
ConvertMetricFamily/OM2.0/HISTOGRAM-24           2.867µ ± 5%      104.0 ± 0%     5.000 ± 0%
ConvertMetricFamily/TEXT/GAUGE_HISTOGRAM-24      2.261µ ± 4%      160.0 ± 0%     5.000 ± 0%
ConvertMetricFamily/OM1.0/GAUGE_HISTOGRAM-24     2.736µ ± 5%      161.0 ± 0%     6.000 ± 0%
ConvertMetricFamily/OM2.0/GAUGE_HISTOGRAM-24     1.624µ ± 2%      48.00 ± 0%     2.000 ± 0%

[Copilot]

Pull request overview

This PR extends the OpenMetrics 2.0 encoder to support serialization and validation of histogram and gauge-histogram metrics, aligning output formatting with the OM2 structure (inline histogram fields plus optional exemplars), and adjusts exemplar emission rules to allow empty-label exemplars when a timestamp is present.

Changes:

• Implemented OpenMetrics 2.0 serialization/validation for classic, native, dual, and gauge histograms (including spans/buckets encoding and created timestamps where applicable).
• Updated exemplar handling to no longer drop exemplars solely due to having an empty label set (still dropped if missing timestamp), emitting {} for empty labels.
• Added comprehensive golden/output and error-case test coverage for histogram variants in OM2.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
expfmt/openmetrics_2_0_create.go	Implements histogram/gaugehistogram encoding + validation and updates exemplar formatting for empty-label exemplars.
expfmt/openmetrics_2_0_create_test.go	Adds output and error tests covering classic/native/dual/gauge histogram serialization and validation rules.

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