Modification should be made to copies of the returned MD.

C:/Go/pkg/mod/google.golang.org/grpc@v1.29.1/metadata/metadata.go:180

// FromOutgoingContextRaw returns the un-merged, intermediary contents
// of rawMD. Remember to perform strings.ToLower on the keys. The returned
// MD should not be modified. Writing to it may cause races. Modification
// should be made to copies of the returned MD.
//
// This is intended for gRPC-internal use ONLY.
func FromOutgoingContextRaw(ctx context.Context) (MD, [][]string, bool) {
   raw, ok := ctx.Value(mdOutgoingKey{}).(rawMD)
   if !ok {
      return nil, nil, false
   }

   return raw.md, raw.added, true
}

// FromOutgoingContext returns the outgoing metadata in ctx if it exists.  The
// returned MD should not be modified. Writing to it may cause races.
// Modification should be made to copies of the returned MD.
func FromOutgoingContext(ctx context.Context) (MD, bool) {
   raw, ok := ctx.Value(mdOutgoingKey{}).(rawMD)
   if !ok {
      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...))
   }
   return Join(mds...), ok
}