Make Metadata Stringer implementation deterministic

Signed-off-by: Davanum Srinivas <[email protected]>

Author: dims

metadata/metadata.go

@@ -24,6 +24,7 @@ package metadata // import "google.golang.org/grpc/metadata"
 import (
 	"context"
 	"fmt"
+	"sort"
 	"strings"
 
 	"google.golang.org/grpc/internal"
@@ -91,15 +92,19 @@ func Pairs(kv ...string) MD {
 }
 
 // String implements the Stringer interface for pretty-printing a MD.
-// Ordering of the values is non-deterministic as it ranges over a map.
 func (md MD) String() string {
 	var sb strings.Builder
 	fmt.Fprintf(&sb, "MD{")
-	for k, v := range md {
+	keys := make([]string, 0, len(md))
+	for k := range md {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, k := range keys {
 		if sb.Len() > 3 {
 			fmt.Fprintf(&sb, ", ")
 		}
-		fmt.Fprintf(&sb, "%s=[%s]", k, strings.Join(v, ", "))
+		fmt.Fprintf(&sb, "%s=[%s]", k, strings.Join(md[k], ", "))
 	}
 	fmt.Fprintf(&sb, "}")
 	return sb.String()