fix: Tighten OM2 summary and start timestamp output

[zeitlinger]

Summary

Tighten the OpenMetrics 2.0 text writer for non-histogram conformance details:

• emit counter start timestamps inline with st@ instead of a separate _created sample
• always include the quantile composite field for OM2 summaries, including quantile:[]
• emit all summary exemplars attached to the composite summary sample

Context

These are follow-up OM2 conformance fixes separate from native histogram serialization. The existing content negotiation and content-type feature flag behavior is intentionally unchanged.

Validation

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