Configurable head-based sampling with always-sample-on-error

[shaneutt]

Configurable trace sampling rate with always-sample-on-error override.

Requirements

• Head-based sampling with configurable rate (0.0 to 1.0)
• Default: sample 1% of requests (0.01)
• Always-sample-on-error: override sampling decision if request results in error
• Respect incoming sampling decision from traceparent
• Parent-based sampling: inherit parent's sampling decision

Implementation Notes

• Use OpenTelemetry Sampler trait
• Composite sampler: rate-based + error-based override
• Error detection at response phase (5xx status codes)

Parent epic: #10