Now finally after consulting the luametafun and the MetaFun manual I’ve found what I was looking for:

The code:

\startMPcode 
picture p; p := lmt_svg [ filename = "steguppgift.svg", width = 400 ] ;
draw p;
path linje ; linje := (15,-330) -- (280,-90); % adding a missing line in the .svg file
drawarrow linje withcolor black ;
draw thelabel("37°",(160,-330));
draw thelabel("$5,0$m",(150,-160));
\stopMPcode