luamplib figure caption

\documentclass{article}
\usepackage{luamplib}
\usepackage{lipsum}
% 讓圖檔置中以便配合 LaTeX figure 環境的 caption 位置
\everymplib{%
  verbatimtex
    \center\leavevmode
  etex;
  beginfig(1);}
\everyendmplib{%
  verbatimtex
    \endcenter
  etex;
  endfig;}

\begin{document}

\lipsum[1]

\begin{figure}
\begin{mplibcode}
  draw unitsquare scaled 70;
  draw (10,20);
  draw (10,15) scaled 2;
  draw (30,40) withpen pencircle scaled 4;
  pickup pencircle scaled 8;
  draw (40,50);
  pickup pencircle scaled 10;
  draw (50,60);
\end{mplibcode}
\caption{Luamplib test.}
\end{figure}

\lipsum[2-3]

\end{document}