fix: Fix bug in BytesToTraceID, add test for it. (#182)

## Which problem is this PR solving?

- The bug fix in #179 had a bug, and it was found by a test in an
internal library before deploy.

## Short description of the changes

- Fix bug in BytesToTraceID
- Add a test that would have found the bug, and more test cases for the
new stuff

This requires a release to v0.22.1.

otlp/traces.go

@@ -224,10 +224,11 @@ func BytesToTraceID(traceID []byte) string {
 		// The spec says that traceID and spanID should be encoded as hex, but
 		// the protobuf system is interpreting them as b64, so we need to
 		// reverse them back to b64 and then reencode as hex.
-		encoded := make([]byte, base64.StdEncoding.EncodedLen(len(traceID)))
+		encoded = make([]byte, base64.StdEncoding.EncodedLen(len(traceID)))
 		base64.StdEncoding.Encode(encoded, traceID)
 	default:
 		encoded = make([]byte, len(traceID)*2)
+		hex.Encode(encoded, traceID)
 	}
 	return string(encoded)
 }

otlp/traces_test.go

@@ -2,6 +2,7 @@ package otlp
 
 import (
 	"bytes"
+	"encoding/base64"
 	"encoding/hex"
 	"io"
 	"math"
@@ -1063,3 +1064,63 @@ func TestKnownInstrumentationPrefixesReturnTrue(t *testing.T) {
 		})
 	}
 }
+
+func Test_BytesToTraceID(t *testing.T) {
+	tests := []struct {
+		name    string
+		traceID string
+		b64     bool
+		want    string
+	}{
+		{
+			name:    "64-bit traceID",
+			traceID: "cbe4decd12429177",
+			want:    "cbe4decd12429177",
+		},
+		{
+			name:    "128-bit zero-padded traceID",
+			traceID: "0000000000000000cbe4decd12429177",
+			want:    "cbe4decd12429177",
+		},
+		{
+			name:    "128-bit non-zero-padded traceID",
+			traceID: "f23b42eac289a0fdcde48fcbe3ab1a32",
+			want:    "f23b42eac289a0fdcde48fcbe3ab1a32",
+		},
+		{
+			name:    "Non-hex traceID",
+			traceID: "foobar1",
+			want:    "666f6f62617231",
+		},
+		{
+			name:    "Longer non-hex traceID",
+			traceID: "foobarbaz",
+			want:    "666f6f62617262617a",
+		},
+		{
+			name:    "traceID munged by browser",
+			traceID: "6e994e8673e93a51200c137330aeddad",
+			b64:     true,
+			want:    "6e994e8673e93a51200c137330aeddad",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var traceID []byte
+			var err error
+			if tt.b64 {
+				traceID, err = base64.StdEncoding.DecodeString(tt.traceID)
+			} else {
+				traceID, err = hex.DecodeString(tt.traceID)
+			}
+			if err != nil {
+				traceID = []byte(tt.traceID)
+			}
+			got := BytesToTraceID(traceID)
+			if got != tt.want {
+				t.Errorf("got:  %#v\n\twant: %#v", got, tt.want)
+			}
+		})
+	}
+}