this is a boxed u:label

basically it gets the string as picture,

its bounding box corners, and define the x and y to draw the box, 

then draw the string picture

point 10 20 u:label -attr val 20

def p_u_label ( expr p,r,s,t ) =

  picture pic;

  pic := thelabel( ATTR_val, (0,0)) scaled s;

 pair q[]; numeric n[], m[], v;

 path bb;

 v := defaultscale;

 qO = ulcorner pic;

 q1 = lrcorner pic;

 nO := ypart q0 + 2.5 * v;

 n1 := ypart q1 - 1.5*v;

 m0 := xpart q0 - v;

 m1 := xpart q1 + 1.5*v;

bb := (m0,n0) -- (m0,n1) -- (m1,n1) -- (m1,n0) -- cycle;

draw bb  rotated r shifted p;

draw pic rotated r shifted p;

enddef