code: func getUserAgentFromContext(ctx context.Context) string {
  	var upstreamUA string
  	if ctx != nil {
  		var ki interface{} = ctx.Value(UAStringKey{})
  		if ki != nil {
  			upstreamUA = ctx.Value(UAStringKey{}).(string)
  		}
  	}
  	return upstreamUA
  }
docs: // getUserAgentFromContext returns the previously saved user-agent context stored in ctx, if one exists