How to define at tikz style option to draw a dimension line between to specific points

3nd EDIT: I completely switched gears and just present a solution that is almost entirely taken from this cool answer. It allows you to control the distance of the measurement bar. The only thing I added is to store \pgfdecoratedpathlength in a macro that I use in the annotation. UPDATE: Changed the style such that by default no line is drawn.

\documentclass{article} % mostly taken from https://tex.stackexchange.com/a/37926/121799
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.markings,decorations.text}

\begin{document}
 \pgfkeys{/pgf/decoration/.cd,
      distance/.initial=10pt
} 

\pgfdeclaredecoration{add dim}{final}{
\state{final}{% 
\pgfmathsetmacro{\dist}{5pt*\pgfkeysvalueof{/pgf/decoration/distance}/abs(\pgfkeysvalueof{/pgf/decoration/distance})}    
          \pgfpathmoveto{\pgfpoint{0pt}{0pt}}             
          \pgfpathlineto{\pgfpoint{0pt}{2*\dist}}   
          \pgfpathmoveto{\pgfpoint{\pgfdecoratedpathlength}{0pt}} 
          \pgfpathlineto{\pgfpoint{(\pgfdecoratedpathlength}{2*\dist}}     
          \pgfsetarrowsstart{latex}
          \pgfsetarrowsend{latex}
          \pgfpathmoveto{\pgfpoint{0pt}{\dist}}
          \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{\dist}} 
          \pgfusepath{stroke} 
          \pgfpathmoveto{\pgfpoint{0pt}{0pt}}
          \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{0pt}}
          \pgfextra{
            \pgfmathtruncatemacro{\MyLen}{round(10*\pgfdecoratedpathlength/28.45369)}
            \xdef\NewLen{\MyLen}
            }
}}

\tikzset{dim/.style args={#1}{draw=none,decoration={add dim,distance=#1},
                decorate,
                postaction={decorate,decoration={text along path,
                                                 raise=#1,
                                                 text align={align=center},
                                                 text={|\sf|$L=\NewLen$mm{}}}}}}


\begin{tikzpicture}
\coordinate (A) at (0,0);
\coordinate (B) at (4,2);
\coordinate (C) at (8,-2);

\draw[dim={10pt}]  (A) --  (B);
\draw[dim={-15pt},draw]  (B) --  (C); 

\draw[fill=gray] (A) circle(2pt); 
\draw[fill=gray] (B) circle(2pt);
\draw[fill=gray] (C) circle(2pt);    
\end{tikzpicture}

\end{document}

Whit this Dimensioning of a technical drawing in TikZ and a little add one can do :

If the cotation parameter is empty, the macro displays the real length.

\documentclass{article}
\usepackage{xparse,tikz}
\usetikzlibrary{calc}
\RequirePackage{ifluatex}

\makeatletter

\ifluatex
\RequirePackage{pdftexcmds}
\let\pdfstrcmp\pdf@strcmp
\let\pdffilemoddate\pdf@filemoddate
\fi

\tikzset{%
    Cote node/.style={%
        midway,
        %sloped,
        fill=white,
        inner sep=1.5pt,
        outer sep=2pt
    },
    Cote arrow/.style={%
        <->,
        >=latex,
        very thin
    }
}

\makeatletter
\NewDocumentCommand{\Cote}{%
    s       % cotation avec les flèches à l'extérieur
    D<>{1.5pt} % offset des traits
    O{.75cm}    % offset de cotation
    m       % premier point
    m       % second point
    m       % étiquette
    D<>{o}  % () coordonnées -> angle
            % h -> horizontal,
            % v -> vertical
            % o or what ever -> oblique
    O{}     % parametre du tikzset
    }{%

    {\tikzset{#8}

    \ifx\hfuzz#6\hfuzz
        \coordinate (@a) at #4 ;
        \coordinate (@b) at #5 ;
        \pgfpointdiff{\pgfpointanchor{@a}{center}}{\pgfpointanchor{@b}{center}}
        % no need to use a new dimen
        \pgf@xa=\pgf@x
        \pgf@ya=\pgf@y
        % to convert from pt to cm   
        \pgfmathsetmacro{\@et}{veclen(\pgf@xa,\pgf@ya)/28.45274}
    \else
        \edef\@et{#6}
    \fi

    \coordinate (@1) at #4 ;
    \coordinate (@2) at #5 ;

    \if #7H % Cotation traits horizontaux
        \coordinate (@0) at ($($#4!.5!#5$) + (#3,0)$) ; 
        \coordinate (@5) at ($#5+(#3,0)$) ;
        \coordinate (@4) at ($#4+(#3,0)$) ;
    \else
    \if #7V % Cotation traits verticaux
        \coordinate (@0) at ($($#4!.5!#5$) + (#3,0)$) ; 
        \coordinate (@5) at ($#5+(0,#3)$) ;
        \coordinate (@4) at ($#4+(0,#3)$) ;
    \else
    \if #7v % Cotation verticale
        \coordinate (@0) at ($($#4!.5!#5$) + (#3,0)$) ; 
        \coordinate (@4) at (@0|-@1) ;
        \coordinate (@5) at (@0|-@2) ;
    \else
    \if #7h % Cotation horizontale
        \coordinate (@0) at ($($#4!.5!#5$) + (0,#3)$) ; 
        \coordinate (@4) at (@0-|@1) ;
        \coordinate (@5) at (@0-|@2) ;
    \else % cotation encoche
    \ifnum\pdfstrcmp{\unexpanded\expandafter{\@car#7\@nil}}{(}=\z@
        \coordinate (@5) at ($#7!#3!#5$) ;
        \coordinate (@4) at ($#7!#3!#4$) ;
    \else % cotation oblique    
        \coordinate (@5) at ($#5!#3!90:#4$) ;
        \coordinate (@4) at ($#4!#3!-90:#5$) ;
    \fi\fi\fi\fi\fi

    \draw[very thin,shorten >= #2,shorten <= -2*#2] (@4) -- #4 ;
    \draw[very thin,shorten >= #2,shorten <= -2*#2] (@5) -- #5 ;

    \IfBooleanTF #1 {% avec étoile
    \draw[Cote arrow,-] (@4) -- (@5)
        node[Cote node] {\@et\strut};
    \draw[Cote arrow,<-] (@4) -- ($(@4)!-6pt!(@5)$) ;   
    \draw[Cote arrow,<-] (@5) -- ($(@5)!-6pt!(@4)$) ;   
    }{% sans étoile
    \ifnum\pdfstrcmp{\unexpanded\expandafter{\@car#7\@nil}}{(}=\z@
        \draw[Cote arrow] (@5) to[bend right]
            node[Cote node] {\@et\strut} (@4) ;
    \else
    \draw[Cote arrow] (@4) -- (@5)
        node[Cote node] {\@et\strut};
    \fi
    }}
    }

\makeatother

\begin{document}
\begin {tikzpicture}[font=\sf]
\fill (0,0) coordinate (A) circle (1pt) node[anchor=north west] {A};
\fill (2,3) coordinate (B) circle (1pt) node[anchor=north west] {B};

\Cote[.3]{(B)}{(A)}{}[Cote node/.append style={sloped}]
\Cote[.1]{(B)}{(A)}{L=10}[Cote node/.append style={sloped}]

\end {tikzpicture}
\end{document}

I start a new answer because I made a pgfkey frontend to the cotation macro. In the example I put syntaxes in parallel.

\documentclass{article}
\usepackage{xparse,tikz}
\usetikzlibrary{calc}

\RequirePackage{pdftexcmds}
\makeatletter
\let\pdfstrcmp\pdf@strcmp
\let\pdffilemoddate\pdf@filemoddate

\tikzset{%
    Cote node/.style={%
        midway,
        %sloped,
        fill=white,
        inner sep=1.5pt,
        outer sep=2pt
    },
    Cote arrow/.style={%
        <->,
        >=latex,
        very thin
    },
    Cote lines/.style={%
        very thin
    },
    Cote/.style={to path={\pgfextra{
        \pgfinterruptpath
        \ifCoteOut
        \expandafter\Cote\expandafter*\expandafter[\expandafter\@offset\expandafter]\expandafter{\expandafter(\expandafter\tikztostart\expandafter)\expandafter}\expandafter{\expandafter(\expandafter\tikztotarget\expandafter)\expandafter}\expandafter{\expandafter\@text\expandafter}\expandafter<\@aspect>
        \else
        \expandafter\Cote\expandafter[\expandafter\@offset\expandafter]\expandafter{\expandafter(\expandafter\tikztostart\expandafter)\expandafter}\expandafter{\expandafter(\expandafter\tikztotarget\expandafter)\expandafter}\expandafter{\expandafter\@text\expandafter}\expandafter<\@aspect>
        \fi
        \endpgfinterruptpath
        }(\tikztostart) -- (\tikztotarget) \tikztonodes}}
}

\newif\ifCoteOut

\pgfkeys{tikz/Cote/.cd,
    offset/.store in=\@offset,
    offset=.75cm,
    text/.store in=\@text,
    text=,
    aspect/.store in=\@aspect,
    aspect=o,
    Cote out/.is if=CoteOut,
    Cote out=false,
} 
\makeatletter

\NewDocumentCommand{\Cote}{%
    s       % cotation avec les flèches à l'extérieur
    D<>{1.5pt} % offset des traits
    O{.75cm}    % offset de cotation
    m       % premier point
    m       % second point
    m       % étiquette
    D<>{o}  % () coordonnées -> angle
            % h -> horizontal,
            % v -> vertical
            % o or what ever -> oblique
    O{}     % parametre du tikzset
    }{%

    {\tikzset{#8}

    \coordinate (@1) at #4 ;
    \coordinate (@2) at #5 ;

    \if #7H % Cotation traits horizontaux
        \coordinate (@0) at ($($#4!.5!#5$) + (#3,0)$) ; 
        \coordinate (@5) at ($#5+(#3,0)$) ;
        \coordinate (@4) at ($#4+(#3,0)$) ;
    \else
    \if #7V % Cotation traits verticaux
        \coordinate (@0) at ($($#4!.5!#5$) + (#3,0)$) ; 
        \coordinate (@5) at ($#5+(0,#3)$) ;
        \coordinate (@4) at ($#4+(0,#3)$) ;
    \else
    \if #7v % Cotation verticale
        \coordinate (@0) at ($($#4!.5!#5$) + (#3,0)$) ; 
        \coordinate (@4) at (@0|-@1) ;
        \coordinate (@5) at (@0|-@2) ;
    \else
    \if #7h % Cotation horizontale
        \coordinate (@0) at ($($#4!.5!#5$) + (0,#3)$) ; 
        \coordinate (@4) at (@0-|@1) ;
        \coordinate (@5) at (@0-|@2) ;
    \else % cotation encoche
    \ifnum\pdfstrcmp{\unexpanded\expandafter{\@car#7\@nil}}{(}=\z@
        \coordinate (@5) at ($#7!#3!#5$) ;
        \coordinate (@4) at ($#7!#3!#4$) ;
    \else % cotation oblique    
        \coordinate (@5) at ($#5!#3!90:#4$) ;
        \coordinate (@4) at ($#4!#3!-90:#5$) ;
    \fi\fi\fi\fi\fi

    \draw[Cote lines,shorten >= #2,shorten <= -2*#2] (@4) -- #4 ;
    \draw[Cote lines,shorten >= #2,shorten <= -2*#2] (@5) -- #5 ;

    \IfBooleanTF #1 {% avec étoile
    \draw[Cote arrow,-] (@4) -- (@5) node[Cote node] {#6\strut};
    \draw[Cote arrow,<-] (@4) -- ($(@4)!-6pt!(@5)$) ;   
    \draw[Cote arrow,<-] (@5) -- ($(@5)!-6pt!(@4)$) ;   
    }{% sans étoile
    \ifnum\pdfstrcmp{\unexpanded\expandafter{\@car#7\@nil}}{(}=\z@
        \draw[Cote arrow] (@5) to[bend right]
            node[Cote node] {#6\strut} (@4) ;
    \else
    \draw[Cote arrow] (@4) -- (@5) node[Cote node] {#6\strut};
    \fi
    }}
    }

\makeatother

\begin{document}
\begin {tikzpicture}[font=\sf]
\small
\draw[thick,blue,fill=blue!25]
        (0,1) coordinate (A)
    --  (3,1) coordinate (B)
    --  (5,2) coordinate (C)
    --  (5,4) coordinate (D)
    --  (3,4) coordinate (E)
    --  (2.5,3) coordinate (F)
    --  (2,4) coordinate (G)
    --  (0,4) coordinate (H)
    --cycle ;

\draw[red,fill=red!25] (2.5,3.9) circle (.39) ;

% macro version
%\Cote{(A)}{(B)}{1}

\path[Cote/.cd,
text={1},           % cotation text - distance if empty
] (A) to[Cote] (B) ;

%\Cote{(B)}{(C)}{2}[red] % macro version

\path[red,  % pass general options to local tikzset
Cote/.cd,
text={2},
] (B) to[Cote] (C) ;

% macro version
%\Cote[.3cm]{(B)}{(C)}{2}[%
%   red,Cote node/.append style={sloped}]]

\path[red,Cote node/.append style={sloped},
Cote/.cd,
offset=.3cm,        % change offset
text={2},
] (B) to[Cote] (C) ;

%\Cote{(B)}{(C)}{2 bis}<h>[Cote node/.append style={fill=blue!25}]

\path[red,Cote node/.append style={fill=blue!25},
Cote/.cd,
text={2 bis},
aspect=h,   % h horizontal node even if path not
] (B) to[Cote] (C) ;

% macro version
%\Cote[.7cm]{(C)}{(D)}{3}

\path[Cote/.cd,
offset=.7cm,
text={3},
] (C) to[Cote] (D) ;

% macro version
%\Cote[.3cm]{(C)}{(D)}{3 bis}[%
%       Cote node/.append style={rotate=-90}]

\path[Cote node/.append style={rotate=90},
Cote/.cd,
offset=.3cm,
text={3 bis},
] (C) to[Cote] (D) ;

% macro version
%\Cote[1.1cm]{(C)}{(D)}{3 ter}[%
%           Cote node/.append style={right}]

% macro version
\path[Cote node/.append style={right},
Cote/.cd,
offset=1.1cm,
text={3 ter},
] (C) to[Cote] (D) ;

% macro version
%\Cote[2cm]{(G)}{(E)}{?$^\circ$}<(F)>

% #### ###########  problem here
\path[Cote/.cd,
offset=2cm,
aspect=(F), % <- the left parenthesis seem not to bee seen in the test above
text={?$^\circ$},
] (G) to[Cote] (E) ;

% macro version
% \Cote*[1.2cm]{(2.11,3.9)}{(2.89,3.9)}{4}[
% Cote node/.append style={left=.6cm,fill=blue!25}]

\path[Cote node/.append style={left=.6cm,fill=blue!25},
Cote/.cd,
offset=1.2cm,
text={4},
Cote out=true,  % external cotation
] (2.11,3.9) to[Cote] (2.89,3.9) ;

% macro version
%\Cote[-2cm]{(A)}{(2.5,4.29)}{5}<v>

\path[Cote/.cd,
offset=-2cm,
text={5},
aspect=v,
] (A) to[Cote] (2.5,4.29) ;


\begin{scope}[yshift=-5cm]
\draw[thick,blue,fill=blue!25]
    (0,0) coordinate (A)
--  (2,2) coordinate (B)
--  (2,4) coordinate (C)
--  (0,2) coordinate (D)
--cycle ;
\end{scope}

%\Cote[.5]{(A)}{(B)}{6}<H>
%\Cote[.5]{(D)}{(C)}{7}<V>

\path[Cote/.cd,
offset=.5cm,
text={6},
aspect=H,
] (A) to[Cote] (B) ;

\path[Cote/.cd,
offset=.5cm,
text={6},
aspect=V,
] (D) to[Cote] (C) ;

\end {tikzpicture}
\end{document}