code: func AddPrefix(ctx context.Context, prefix string) context.Context {
  	if prefix == "" {
  		return ctx
  	}
  	pfx := Prefix(ctx)
  	if pfx == "" {
  		return WithPrefix(ctx, prefix)
  	}
  	return WithPrefix(ctx, pfx+prefix)
  }
docs: // AddPrefix returns a context with the given prefix added to end of the
  // existing prefix