TraceID.String prefix with zeroes

The wire encoding of the TraceID uses zero prefixes (jaegertracing#472).
The JaegerUI also uses zero prefixes (since 1.16).
So it makes sense that the Stringer of the TraceID also does this.

Resolves jaegertracing#532

Signed-off-by: Lukas Vogel <vogel@anapaya.net>

span_context.go

@@ -344,9 +344,9 @@ func (c *SpanContext) isDebugIDContainerOnly() bool {
 
 func (t TraceID) String() string {
 	if t.High == 0 {
-		return fmt.Sprintf("%x", t.Low)
+		return fmt.Sprintf("%016x", t.Low)
 	}
-	return fmt.Sprintf("%x%016x", t.High, t.Low)
+	return fmt.Sprintf("%016x%016x", t.High, t.Low)
 }
 
 // TraceIDFromString creates a TraceID from a hexadecimal string

span_context_test.go

@@ -15,6 +15,7 @@
 package jaeger
 
 import (
+	"math"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -166,3 +167,28 @@ func TestSpanContext_CopyFrom(t *testing.T) {
 	assert.Equal(t, ctx, ctx2)
 	assert.Equal(t, "y", ctx2.baggage["x"])
 }
+
+func TestTraceIDString(t *testing.T) {
+	var tests = map[string]struct {
+		in       TraceID
+		expected string
+	}{
+		"Empty TraceID": {
+			in:       TraceID{},
+			expected: "0000000000000000",
+		},
+		"TraceID low only": {
+			in:       TraceID{Low: math.MaxUint64/16 - 405},
+			expected: "0ffffffffffffe6a",
+		},
+		"TraceID low and high": {
+			in:       TraceID{High: math.MaxUint64 / 16, Low: math.MaxUint64/16 - 405},
+			expected: "0fffffffffffffff0ffffffffffffe6a",
+		},
+	}
+	for name, tc := range tests {
+		t.Run(name, func(t *testing.T) {
+			assert.Equal(t, tc.expected, tc.in.String())
+		})
+	}
+}