feat: support customized header to metadata processor (#2162)

* chore: add more tests

* feat: support customized header processor

gateway/internal/headerprocessor_test.go

@@ -0,0 +1,21 @@
+package internal
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBuildHeadersNoValue(t *testing.T) {
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Add("a", "b")
+	assert.Nil(t, ProcessHeaders(req.Header))
+}
+
+func TestBuildHeadersWithValues(t *testing.T) {
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Add("grpc-metadata-a", "b")
+	req.Header.Add("grpc-metadata-b", "b")
+	assert.ElementsMatch(t, []string{"gateway-A:b", "gateway-B:b"}, ProcessHeaders(req.Header))
+}