Fix HTTP headers not being canonicalized in tracing

pkg/tracing/tracing.go

@@ -127,8 +127,8 @@ func NewTracer(tracer trace.Tracer, capturedRequestHeaders, capturedResponseHead
 	return &Tracer{
 		Tracer:                  tracer,
 		safeQueryParams:         safeQueryParams,
-		capturedRequestHeaders:  capturedRequestHeaders,
-		capturedResponseHeaders: capturedResponseHeaders,
+		capturedRequestHeaders:  canonicalizeHeaders(capturedRequestHeaders),
+		capturedResponseHeaders: canonicalizeHeaders(capturedResponseHeaders),
 	}
 }
 
@@ -346,3 +346,18 @@ func defaultStatus(code int) (codes.Code, string) {
 	}
 	return codes.Unset, ""
 }
+
+// canonicalizeHeaders converts a slice of header keys to their canonical form.
+// It uses http.CanonicalHeaderKey to ensure that the headers are in a consistent format.
+func canonicalizeHeaders(headers []string) []string {
+	if headers == nil {
+		return nil
+	}
+
+	canonicalHeaders := make([]string, len(headers))
+	for i, header := range headers {
+		canonicalHeaders[i] = http.CanonicalHeaderKey(header)
+	}
+
+	return canonicalHeaders
+}

pkg/tracing/tracing_test.go

@@ -17,6 +17,7 @@ import (
 	"github.com/traefik/traefik/v3/pkg/types"
 	"go.opentelemetry.io/collector/pdata/ptrace/ptraceotlp"
 	"go.opentelemetry.io/otel/trace"
+	"go.opentelemetry.io/otel/trace/noop"
 )
 
 func Test_safeFullURL(t *testing.T) {
@@ -414,3 +415,52 @@ func TestTracerProvider(t *testing.T) {
 	span.TracerProvider().Tracer("github.com/traefik/traefik")
 	span.TracerProvider().Tracer("other")
 }
+
+// TestNewTracer_HeadersCanonicalization tests that NewTracer properly canonicalizes headers.
+func TestNewTracer_HeadersCanonicalization(t *testing.T) {
+	testCases := []struct {
+		desc                     string
+		inputHeaders             []string
+		expectedCanonicalHeaders []string
+	}{
+		{
+			desc:                     "Empty headers",
+			inputHeaders:             []string{},
+			expectedCanonicalHeaders: []string{},
+		},
+		{
+			desc:                     "Already canonical headers",
+			inputHeaders:             []string{"Content-Type", "User-Agent", "Accept-Encoding"},
+			expectedCanonicalHeaders: []string{"Content-Type", "User-Agent", "Accept-Encoding"},
+		},
+		{
+			desc:                     "Lowercase headers",
+			inputHeaders:             []string{"content-type", "user-agent", "accept-encoding"},
+			expectedCanonicalHeaders: []string{"Content-Type", "User-Agent", "Accept-Encoding"},
+		},
+		{
+			desc:                     "Mixed case headers",
+			inputHeaders:             []string{"CoNtEnT-tYpE", "uSeR-aGeNt", "aCcEpT-eNcOdInG"},
+			expectedCanonicalHeaders: []string{"Content-Type", "User-Agent", "Accept-Encoding"},
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+
+			// Create a mock tracer using a no-op tracer from OpenTelemetry
+			mockTracer := noop.NewTracerProvider().Tracer("test")
+
+			// Test capturedRequestHeaders
+			tracer := NewTracer(mockTracer, test.inputHeaders, nil, nil)
+			assert.Equal(t, test.expectedCanonicalHeaders, tracer.capturedRequestHeaders)
+			assert.Nil(t, tracer.capturedResponseHeaders)
+
+			// Test capturedResponseHeaders
+			tracer = NewTracer(mockTracer, nil, test.inputHeaders, nil)
+			assert.Equal(t, test.expectedCanonicalHeaders, tracer.capturedResponseHeaders)
+			assert.Nil(t, tracer.capturedRequestHeaders)
+		})
+	}
+}