feat: Add OM2 native histogram text output

[zeitlinger]

Summary

Adds OpenMetrics 2.0 text output for native histograms behind the nativeHistograms flag.

• serializes native histogram fields (schema, zero_threshold, zero_count, native spans/buckets)
• supports native gauge histograms with gcount/gsum
• supports mixed native + classic histograms with classic bucket output last
• emits all histogram exemplars on composite/native histogram samples
• adds unit/integration coverage for native histogram content negotiation and edge cases

Validation

• ./mvnw test -pl prometheus-metrics-exposition-textformats -Dtest=OpenMetrics2TextFormatWriterTest,ExpositionFormatsTest -Dcoverage.skip=true -Dcheckstyle.skip=true
• mise run build