Specify characters allowed in metadata keys (#1299)
diff --git a/metadata/metadata.go b/metadata/metadata.go
index b6466c7..4bf6c52 100644
--- a/metadata/metadata.go
+++ b/metadata/metadata.go
@@ -37,7 +37,13 @@
type MD map[string][]string
// New creates an MD from a given key-value map.
-// Keys are automatically converted to lowercase.
+//
+// Only the following ASCII characters are allowed in keys:
+// - digits: 0-9
+// - uppercase letters: A-Z (normalized to lower)
+// - lowercase letters: a-z
+// - special characters: -_.
+// Uppercase letters are automatically converted to lowercase.
func New(m map[string]string) MD {
md := MD{}
for k, val := range m {
@@ -49,7 +55,13 @@
// Pairs returns an MD formed by the mapping of key, value ...
// Pairs panics if len(kv) is odd.
-// Keys are automatically converted to lowercase.
+//
+// Only the following ASCII characters are allowed in keys:
+// - digits: 0-9
+// - uppercase letters: A-Z (normalized to lower)
+// - lowercase letters: a-z
+// - special characters: -_.
+// Uppercase letters are automatically converted to lowercase.
func Pairs(kv ...string) MD {
if len(kv)%2 == 1 {
panic(fmt.Sprintf("metadata: Pairs got the odd number of input pairs for metadata: %d", len(kv)))
