Implement trace context propagators

This commit implements the trace-context propagation logic, and allows
for the creation of trace context headers in the W3C, B3 (openzipkin),
and Jaeger formats.

Note that a lot of the code and logic present in this commit are
inherited and refactored from the xk6-distributed-tracing project.

js/modules/k6/experimental/tracing/encoding.go

@@ -0,0 +1,19 @@
+package tracing
+
+import (
+	"math/rand"
+)
+
+// randHexStringRunes returns a random string of n hex characters.
+//
+// Note that this function uses a non-cryptographic random number generator.
+func randHexString(n int) string {
+	hexRunes := []rune("123456789abcdef")
+
+	b := make([]rune, n)
+	for i := range b {
+		b[i] = hexRunes[rand.Intn(len(hexRunes))] //nolint:gosec
+	}
+
+	return string(b)
+}

js/modules/k6/experimental/tracing/propagator.go

@@ -0,0 +1,90 @@
+package tracing
+
+import (
+	"net/http"
+)
+
+// Propagator is an interface for trace context propagation
+type Propagator interface {
+	Propagate(traceID string) (http.Header, error)
+}
+
+const (
+	// W3CPropagatorName is the name of the W3C trace context propagator
+	W3CPropagatorName = "w3c"
+
+	// W3CHeaderName is the name of the W3C trace context header
+	W3CHeaderName = "traceparent"
+
+	// W3CVersion is the version of the supported W3C trace context header.
+	// The current specification assumes the version is set to 00.
+	W3CVersion = "00"
+
+	// W3CUnsampledTraceFlag is the trace-flag value for an unsampled trace.
+	W3CUnsampledTraceFlag = "00"
+
+	// W3CSampledTraceFlag is the trace-flag value for a sampled trace.
+	W3CSampledTraceFlag = "01"
+)
+
+// W3CPropagator is a Propagator for the W3C trace context header
+type W3CPropagator struct{}
+
+// Propagate returns a header with a random trace ID in the W3C format
+func (p *W3CPropagator) Propagate(traceID string) (http.Header, error) {
+	parentID := randHexString(16)
+
+	return http.Header{
+		W3CHeaderName: {
+			W3CVersion + "-" + traceID + "-" + parentID + "-" + W3CSampledTraceFlag,
+		},
+	}, nil
+}
+
+const (
+	// B3PropagatorName is the name of the B3 trace context propagator
+	B3PropagatorName = "b3"
+
+	// B3HeaderName is the name of the B3 trace context header
+	B3HeaderName = B3PropagatorName
+)
+
+// B3Propagator is a Propagator for the B3 trace context header
+type B3Propagator struct{}
+
+// Propagate returns a header with a random trace ID in the B3 format
+func (p *B3Propagator) Propagate(traceID string) (http.Header, error) {
+	parentID := randHexString(8)
+	parentSpanID := "1"
+
+	return http.Header{
+		B3HeaderName: {traceID + "-" + parentID + "-" + parentSpanID},
+	}, nil
+}
+
+const (
+	// JaegerPropagatorName is the name of the Jaeger trace context propagator
+	JaegerPropagatorName = "jaeger"
+
+	// JaegerHeaderName is the name of the Jaeger trace context header
+	JaegerHeaderName = "uber-trace-id"
+
+	// JaegerRootSpanID is the universal span ID of the root span.
+	// Its value is zero, which is described in the Jaeger documentation as:
+	// "0 value is valid and means “root span” (when not ignored)"
+	JaegerRootSpanID = "0"
+)
+
+// JaegerPropagator is a Propagator for the Jaeger trace context header
+type JaegerPropagator struct{}
+
+// Propagate returns a header with a random trace ID in the Jaeger format
+func (p *JaegerPropagator) Propagate(traceID string) (http.Header, error) {
+	spanID := randHexString(8)
+	// flags set to 1 means the span is sampled
+	flags := "1"
+
+	return http.Header{
+		JaegerHeaderName: {traceID + ":" + spanID + ":" + JaegerRootSpanID + ":" + flags},
+	}, nil
+}