feat: compositeValues and exemplarCompliance flags for OM2 writer

[zeitlinger]

Fixes #1989

Summary

• compositeValues=true: Histogram, GaugeHistogram, and Summary are
  written as a single composite-value line per the OM2 spec:

  foo {count:17,sum:324789.3,bucket:[0.1:8,0.25:10,0.5:11,1.0:14,+Inf:17]} 1520879607.789 st@1520430000.123
  bar {count:17,sum:324789.3,quantile:[0.95:123.7,0.99:150.0]}
  

  • Bucket counts are cumulative (same semantics as OM1 _bucket lines)
  • GaugeHistogram uses gcount/gsum per spec (note: the tracking issue
    says "count/sum" but the spec is explicit about gcount/gsum)
  • Created timestamp moves inline as st@; no separate _created line
  • Latest exemplar (if present) is appended inline
  • No separate _bucket, _count, _sum lines emitted
  • When compositeValues=false (default), output is unchanged from before

• exemplarCompliance=true: Exemplars without a timestamp are silently
  dropped. The OM2 spec mandates that all exemplars MUST have a timestamp.
  The 128-char LabelSet limit from OM1 was never enforced in the writer, so
  no change needed there.

Test plan

• testCompositeHistogram — basic single-line histogram output
• testCompositeHistogramWithLabelsTimestampAndCreated — labels, scrape
  timestamp, and st@ start timestamp
• testCompositeHistogramWithExemplar — exemplar appended inline
• testCompositeGaugeHistogram — gcount/gsum fields
• testCompositeSummary — basic single-line summary output
• testCompositeSummaryWithCreatedAndExemplar — st@ + exemplar
• testExemplarComplianceSkipsExemplarWithoutTimestamp — compliance mode
  drops exemplars without timestamps; default mode still emits them
• All 22 existing tests pass unchanged (non-composite path unaffected)