metadata: reduce memory footprint in FromOutgoingContext

When Looking at memory profiles for cockroachdb/cockroach, we observed
that the intermediate metadata.MD array constructed to iterate over
appended metadata escaped to the heap. Fortunately, this is easily
rectifiable.

  go build -gcflags '-m' google.golang.org/grpc/metadata
  ...
  google.golang.org/grpc/metadata/metadata.go:198:13: make([]MD, 0, len(raw.added) + 1) escapes to heap

metadata/metadata.go

@@ -195,12 +195,22 @@ func FromOutgoingContext(ctx context.Context) (MD, bool) {
 		return nil, false
 	}
 
-	mds := make([]MD, 0, len(raw.added)+1)
-	mds = append(mds, raw.md)
-	for _, vv := range raw.added {
-		mds = append(mds, Pairs(vv...))
+	out := raw.md.Copy()
+	for _, added := range raw.added {
+		if len(added)%2 == 1 {
+			panic(fmt.Sprintf("metadata: FromOutgoingContext got the odd number of input pairs for metadata: %d", len(added)))
+		}
+
+		var key string
+		for i, s := range added {
+			if i%2 == 0 {
+				key = strings.ToLower(s)
+				continue
+			}
+			out[key] = append(out[key], s)
+		}
 	}
-	return Join(mds...), ok
+	return out, ok
 }
 
 type rawMD struct {