sdk/trace: TraceIDRatioBased sampler with tracestate support

[yuanyuanzhao3]

Description

Implements the TraceIDRatioBased sampler with OpenTelemetry tracestate support per the TraceState: Probability Sampling specification.

Part of #7928 (Support TraceIdRatioBased Sampler).

Changes

TraceIDRatioBased sampler

• Tracestate ot key: When sampling, encodes the rejection threshold in the th sub-key (e.g. th:8 for 50% sampling).
• Randomness sources: Uses explicit randomness from the rv sub-key when present; otherwise uses the least-significant 56 bits of the TraceID (per W3C Trace Context Level 2).
• Threshold erasure: When the trace ID is not marked random (FlagsRandom), the sampler erases the th value in tracestate, as required for unknown sampling probability.

AlwaysOn sampler

• Updates tracestate with th:0 when sampling to indicate 100% sampling probability.

Testing

• TestTracestateRandomness: Parsing of the rv sub-key.
• TestEraseTraceStateThKeyValue: Erasure of th in all positions.
• TestInsertOrUpdateTraceStateThKeyValue: Insert/update of th.
• TestTraceIDRatioSamplerShouldSample: Sampling behavior with tracestate.
• TestTracestateIsPassed: Tracestate propagation across samplers.
• TestTraceIdRatioSamplesInclusively: TraceIDRatioBased inclusion property.

Related

• OTEP 235
• OTEP 250

Co-Author

Joshua MacDonald jmacd@users.noreply.github.com