refactor(extproc): enhance utilities and testing infrastructure

- Update utility functions to support image generation processing
- Add comprehensive mock implementations for testing
- Improve test infrastructure for image generation components

Signed-off-by: Hrushikesh Patil <[email protected]>

Author: nutanix-Hrushikesh

internal/extproc/mocks_test.go

@@ -396,3 +396,55 @@ type mockBackendAuthHandler struct{}
 func (m *mockBackendAuthHandler) Do(context.Context, map[string]string, *extprocv3.HeaderMutation, *extprocv3.BodyMutation) error {
 	return nil
 }
+
+// mockImageGenerationMetrics implements [metrics.ImageGenerationMetrics] for testing.
+type mockImageGenerationMetrics struct {
+	requestSuccessCount int
+	requestErrorCount   int
+	model               string
+	backend             string
+	tokenUsageCount     int
+}
+
+func (m *mockImageGenerationMetrics) StartRequest(map[string]string) {}
+func (m *mockImageGenerationMetrics) SetModel(requestModel, responseModel string) {
+	m.model = responseModel
+}
+func (m *mockImageGenerationMetrics) SetBackend(b *filterapi.Backend) { m.backend = b.Name }
+func (m *mockImageGenerationMetrics) RecordTokenUsage(_ context.Context, _ uint32, _ uint32, _ map[string]string) {
+	m.tokenUsageCount++
+}
+func (m *mockImageGenerationMetrics) RecordRequestCompletion(_ context.Context, success bool, _ map[string]string) {
+	if success {
+		m.requestSuccessCount++
+	} else {
+		m.requestErrorCount++
+	}
+}
+func (m *mockImageGenerationMetrics) RecordImageGeneration(_ context.Context, _ int, _ string, _ string, _ map[string]string) {
+}
+
+func (m *mockImageGenerationMetrics) RequireRequestFailure(t *testing.T) {
+	require.Equal(t, 0, m.requestSuccessCount)
+	require.Equal(t, 1, m.requestErrorCount)
+}
+func (m *mockImageGenerationMetrics) RequireRequestNotCompleted(t *testing.T) {
+	require.Equal(t, 0, m.requestSuccessCount)
+	require.Equal(t, 0, m.requestErrorCount)
+}
+func (m *mockImageGenerationMetrics) RequireRequestSuccess(t *testing.T) {
+	require.Equal(t, 1, m.requestSuccessCount)
+	require.Equal(t, 0, m.requestErrorCount)
+}
+func (m *mockImageGenerationMetrics) RequireSelectedModel(t *testing.T, model string) {
+	require.Equal(t, model, m.model)
+}
+func (m *mockImageGenerationMetrics) RequireSelectedBackend(t *testing.T, backend string) {
+	require.Equal(t, backend, m.backend)
+}
+func (m *mockImageGenerationMetrics) RequireTokensRecorded(t *testing.T, count int) {
+	require.Equal(t, count, m.tokenUsageCount)
+}
+
+// Ensure mock implements the interface at compile-time.
+var _ metrics.ImageGenerationMetrics = &mockImageGenerationMetrics{}

internal/extproc/util.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 
+	"github.com/andybalholm/brotli"
 	extprocv3 "github.com/envoyproxy/go-control-plane/envoy/service/ext_proc/v3"
 )
 
@@ -21,7 +22,7 @@ type contentDecodingResult struct {
 }
 
 // decodeContentIfNeeded decompresses the response body based on the content-encoding header.
-// Currently, supports gzip encoding, but can be extended to support other encodings in the future.
+// Currently, supports gzip and brotli encoding, but can be extended to support other encodings in the future.
 // Returns a reader for the (potentially decompressed) body and metadata about the encoding.
 func decodeContentIfNeeded(body []byte, contentEncoding string) (contentDecodingResult, error) {
 	switch contentEncoding {
@@ -34,6 +35,12 @@ func decodeContentIfNeeded(body []byte, contentEncoding string) (contentDecoding
 			reader:    reader,
 			isEncoded: true,
 		}, nil
+	case "br":
+		reader := brotli.NewReader(bytes.NewReader(body))
+		return contentDecodingResult{
+			reader:    reader,
+			isEncoded: true,
+		}, nil
 	default:
 		return contentDecodingResult{
 			reader:    bytes.NewReader(body),