Ykhjuy

MWE:

documentclass[tikz, border=1cm]{standalone}

usepackage{tikz-dimline}

begin{document}

begin{tikzpicture}[

    plotmark/.style = {%

    draw, fill=red, circle, inner sep=0pt, minimum size=4pt

  }

]

    coordinate (A) at (0,0);

    coordinate (B) at (5,3);



    draw (A)--(B);

    dimline {(A)}{(B)}{label};



    node[plotmark, label={above:$A$}] at (A) {};

    node[plotmark, label={above:$B$}] at (B) {};

end{tikzpicture}  

end{document}

Question: How do I position the label and its two arrow lines, to the other end of the gray lines?

This is a pure calc alternative in case dimline doesn't allow you to do that. (I do not know what dimline does.) This is an updated answer in which I try to address your comments. It comes with a decoration indicate dimensions, which has one argument, the label. All distances and other parameters are stored in pgfkeys. Drawing the line plus all the dimension lines and perpendicular lines boils down to

 draw[postaction={indicate dimensions={label}}] (A)--(B);

Here is the code

documentclass[tikz, border=1cm]{standalone}

usetikzlibrary{calc,decorations.pathreplacing}

tikzset{dimlabel distance/.initial=5mm,

vertical lines extend/.initial=5mm,

vertical dim line/.style={gray,thin},

dim arrow line/.style={latex-latex,thin},

dim label/.style={},

indicate dimensions/.style={decorate,decoration={

show path construction,

lineto code={

draw[dim arrow line]

    ($ (tikzinputsegmentfirst)!pgfkeysvalueof{/tikz/dimlabel distance}!-90:(tikzinputsegmentlast) $) -- ($ (tikzinputsegmentlast)!pgfkeysvalueof{/tikz/dimlabel distance}!90:(tikzinputsegmentfirst) $)

    node[midway,sloped,fill=white,dim label]{#1};

draw[vertical dim line] (tikzinputsegmentfirst) -- 

($ (tikzinputsegmentfirst)!pgfkeysvalueof{/tikz/vertical lines extend}!-90:(tikzinputsegmentlast) $)

    (tikzinputsegmentlast) -- 

    ($ (tikzinputsegmentlast)!pgfkeysvalueof{/tikz/vertical lines extend}!90:(tikzinputsegmentfirst) $); 

}}}}



begin{document}

begin{tikzpicture}[

    plotmark/.style = {%

    draw, fill=red, circle, inner sep=0pt, minimum size=4pt

  }

]

    coordinate (A) at (0,0);

    coordinate (B) at (5,3);



    draw[postaction={indicate dimensions={label}}] (A)--(B);



    node[plotmark, label={above:$A$}] at (A) {};

    node[plotmark, label={above:$B$}] at (B) {};

end{tikzpicture}  

end{document}

Initially, both vertical lines extend and dimlabel distance are set to 5mm, such that the gray auxiliary lines terminate where the arrows end.