traceIDRatioSampler: use rightmost bits (#3557)

* use bottom bits

* add changelog entry

Co-authored-by: Chester Cheung <cheung.zhy.csu@gmail.com>

CHANGELOG.md

@@ -24,6 +24,9 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add `Producer` interface and `Reader.RegisterProducer(Producer)` to `go.opentelemetry.io/otel/sdk/metric` to enable external metric Producers. (#3524)
 - Add `NewMetricProducer` to `go.opentelemetry.io/otel/bridge/opencensus`, which can be used to pass OpenCensus metrics to an OpenTelemetry Reader. (#3541)
 - Global logger uses an atomic value instead of a mutex. (#3545)
+- `traceIDRatioSampler` (given by `TraceIDRatioBased(float64)`) now uses the rightmost bits for sampling decisions,
+  fixing random sampling when using ID generators like `xray.IDGenerator`
+  and increasing parity with other language implementations. (#3557)
 
 ### Deprecated
 

sdk/trace/sampling.go

@@ -81,7 +81,7 @@ type traceIDRatioSampler struct {
 
 func (ts traceIDRatioSampler) ShouldSample(p SamplingParameters) SamplingResult {
 	psc := trace.SpanContextFromContext(p.ParentContext)
-	x := binary.BigEndian.Uint64(p.TraceID[0:8]) >> 1
+	x := binary.BigEndian.Uint64(p.TraceID[8:16]) >> 1
 	if x < ts.traceIDUpperBound {
 		return SamplingResult{
 			Decision:   RecordAndSample,