Make Metadata Stringer implementation deterministic

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

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()