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

* chore: add more tests

* feat: support customized header processor

gateway/internal/headerprocessor.go

@@ -0,0 +1,30 @@
+package internal
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+const (
+	metadataHeaderPrefix = "Grpc-Metadata-"
+	metadataPrefix       = "gateway-"
+)
+
+// ProcessHeaders builds the headers for the gateway from HTTP headers.
+func ProcessHeaders(header http.Header) []string {
+	var headers []string
+
+	for k, v := range header {
+		if !strings.HasPrefix(k, metadataHeaderPrefix) {
+			continue
+		}
+
+		key := fmt.Sprintf("%s%s", metadataPrefix, strings.TrimPrefix(k, metadataHeaderPrefix))
+		for _, vv := range v {
+			headers = append(headers, key+":"+vv)
+		}
+	}
+
+	return headers
+}