feat: add LAST9_TRACE_SAMPLE_RATE env var for probabilistic sampling

[prathamesh-sonpatki]

Summary

• Adds LAST9_TRACE_SAMPLE_RATE env var (0.0–1.0) that maps to parentbased_traceidratio sampler
• Takes precedence over OTEL_TRACES_SAMPLER / OTEL_TRACES_SAMPLER_ARG when set
• Simplifies sampling configuration: LAST9_TRACE_SAMPLE_RATE=0.5 → sample 50% of traces

Details

The standard OTel approach requires setting two env vars (OTEL_TRACES_SAMPLER=parentbased_traceidratio + OTEL_TRACES_SAMPLER_ARG=0.5) and understanding sampler types. This adds a single, discoverable alternative.

Precedence: LAST9_TRACE_SAMPLE_RATE > OTEL_TRACES_SAMPLER > default (always_on)

Behavior: Uses parentbased_traceidratio internally, which respects parent sampling decisions — if a parent service already sampled the trace, the child will honor it regardless of the local sample rate.

Invalid values (negative, >1.0, non-numeric) are logged as warnings and ignored, falling back to OTEL_TRACES_SAMPLER.

Test plan

• TestParseSampleRate — validates parsing: empty, 0.0, 0.5, 1.0, negative, >1.0, non-numeric
• TestLoad_SampleRate — validates config loading: unset, set to 0.5, set to 0, invalid
• TestSampleRateOverridesSampler — verifies LAST9_TRACE_SAMPLE_RATE overrides OTEL_TRACES_SAMPLER
• TestSampleRateUnsetFallsBackToSampler — verifies fallback when unset
• Lint clean (golangci-lint run --timeout=5m ./...)

Resolves: FDE-49

🤖 Generated with Claude Code

[ashishl9]

LGTM!

✅ Clean implementation of that simplifies sampling configuration
✅ Proper precedence logic - takes priority over as documented
✅ Comprehensive test coverage - parsing validation, config loading, precedence, and fallback behavior
✅ Clear error handling - invalid values logged and gracefully ignored
✅ Excellent documentation - both in code comments and PR description

The parentbased_traceidratio approach correctly respects parent sampling decisions while allowing local rate control.

[ashishl9]

LGTM!

The implementation is clean and well-tested:

• Simple configuration via LAST9_TRACE_SAMPLE_RATE (0.0-1.0)
• Proper precedence over OTEL_TRACES_SAMPLER
• Comprehensive test coverage for parsing, loading, precedence, and fallback
• Clear error handling for invalid values
• Excellent documentation

The parentbased_traceidratio approach correctly respects parent sampling decisions while allowing local rate control.