code: func NewServerMiddleware(t *zipkin.Tracer, options ...ServerOption) func(http.Handler) http.Handler {
  	return func(next http.Handler) http.Handler {
  		h := &handler{
  			tracer: t,
  			next:   next,
  		}
  		for _, option := range options {
  			option(h)
  		}
  		return h
  	}
  }
docs: // NewServerMiddleware returns a http.Handler middleware with Zipkin tracing.