feat: implement summary support for OpenMetrics 2.0

[dashpole]

Part of #893

Implement serialization and strict validation for summaries in OpenMetrics 2.0 format according to the specification and ABNF grammar.

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/prometheus/common/expfmt
cpu: Intel(R) Xeon(R) CPU @ 2.20GHz
BenchmarkConvertMetricFamily/TEXT/SUMMARY-24         	  345410	      3426 ns/op	      56 B/op	       2 allocs/op
BenchmarkConvertMetricFamily/OM1.0/SUMMARY-24        	  273915	      4486 ns/op	      57 B/op	       3 allocs/op
BenchmarkConvertMetricFamily/OM2.0/SUMMARY-24        	  501536	      2026 ns/op	       0 B/op	       0 allocs/op

[Copilot]

🟡 Changes recommended

The new summary validation rejects negative SampleSum and negative quantile values, which are valid in existing OpenMetrics summary behavior and would cause the OM2.0 encoder to error on legitimate data.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds OpenMetrics 2.0 composite summary support to expfmt by implementing serialization plus validation, and expands the test suite to cover correct formatting and error cases.

Changes:

• Implement writeCompositeSummary for OpenMetrics 2.0 composite summary encoding, including label/quantile/timestamp validation.
• Update OpenMetrics 2.0 encoder documentation comment to reflect newly supported metric types.
• Add comprehensive summary test vectors (success + validation errors + “no partial bytes on error”).

File summaries

File	Description
expfmt/openmetrics_2_0_create.go	Implements OpenMetrics 2.0 composite summary encoding and validation logic.
expfmt/openmetrics_2_0_create_test.go	Adds summary golden tests, validation error tests, and a no-partial-bytes regression test.

Review details

Suppressed comments (1)

expfmt/openmetrics_2_0_create.go:386

• Rejecting negative quantile values for summaries is likely incorrect: quantile values are observed sample values and can be negative (the OpenMetrics v1.0 encoder tests include negative quantile values). This validation would cause the OM2.0 encoder to error on otherwise valid summary data.

		v := q.GetValue()
		if !math.IsNaN(v) && v < 0 {
			return 0, fmt.Errorf("summary quantile value cannot be negative (%g) in metric %s", v, name)
		}

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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[bwplotka]

Looks great, tiny nits only.