documentclass{article}

usepackage{amsmath}

begin{document}

begin{align*}
  d(x, y) &= d(y, x) tag{symmetry} \
          &leq d(y, z) + d(z,x) tag{triangle}
end{align*}

end{document}

documentclass{article}

usepackage{amsmath,xcolor}
makeatletter
defmaketag@@@#1{hbox{m@thnormalfontcolor{black!50}#1}}
makeatother

begin{document}

begin{align*}
  d(x, y) &= d(y, x) tag{symmetry} \
          &leq d(y, z) + d(z,x) tag{triangle}
end{align*}

end{document}