Add metadata.New back

rpc/metadata/metadata.go

@@ -83,9 +83,20 @@ func DecodeKeyValue(k, v string) (string, string, error) {
 	return key, string(val), nil
 }
 
-// MD is a mapping from metadata keys to values.
+// MD is a mapping from metadata keys to values. Users should use the following
+// two convenience functions New and Pairs to generate MD.
 type MD map[string]string
 
+// New creates a MD from given key-value map.
+func New(m map[string]string) MD {
+	md := MD{}
+	for k, v := range m {
+		key, val := encodeKeyValue(k, v)
+		md[key] = val
+	}
+	return md
+}
+
 // Pairs returns an MD formed by the mapping of key, value ...
 // Pairs panics if len(kv) is odd.
 func Pairs(kv ...string) MD {

rpc/metadata/metadata_test.go

@@ -69,7 +69,7 @@ func TestPairsMD(t *testing.T) {
 		md MD
 	}{
 		{[]string{}, MD{}},
-		{[]string{"k1", "v1", "k2", binaryValue}, MD(map[string]string{
+		{[]string{"k1", "v1", "k2", binaryValue}, New(map[string]string{
 			"k1":     "v1",
 			"k2-bin": "woA=",
 		})},