Introduce request-level tracing sampler based on tenant tier #697

Description

Sample enterprise-tier traffic at 100 percent and free-tier at 1 percent, both driven by attributes at span start. Complements tail-based sampling with head-based control.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: internal/tracing/tracing.go
• Use ParentBased(TraceIDRatioBased) with attribute overrides

Suggested execution

• Fork the repo and create a branch
• git checkout -b feat/tenant-aware-sampler
• Implement changes
  • Add internal/tracing/sampler.go
  • Wire into provider setup
  • Metrics for sampled_total by tier
• Validate security and correctness assumptions

Test and commit

• Run tests
  • go test ./internal/tracing/...
• Cover edge cases
  • Missing tier defaults to global ratio
• Include test output and notes

Example commit message

feat: tenant-tier-aware trace sampler

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours