beamer block as wide as the entire page

Perhaps not the best solution, but lying about the \linewidth using a minipage with width \paperwidth does work. The \makebox[\linwidth][c] is to give everything the width of \linewidth and centre-align it.

For the vertical placement you can change the argument given to \vspace*.

\documentclass{beamer}
\usetheme{metropolis}          
\begin{document}
\begin{frame}{Frame}
\centering
\vspace*{5cm}
\metroset{block=fill}
\makebox[\linewidth]{%
  \begin{minipage}{\paperwidth}
    \begin{block}{\vspace*{-3ex}}
    \centering
    A wide block 
    \end{block}
  \end{minipage}
}
 \end{frame}
\end{document}

Looking at the source of the metropolis theme (here), you see that the block environment is made up of several beamercolorbox environments. These take wd={width} as an optional argument to resize them to your desired width. So just add wd=\paperwidth to every occurence of beamercolorbox in the \metropolis@block macro.

The following example shows two things:

• First, I redefine \metropolis@block to have a wd=\metropolis@blockwd as argument for every beamercolorbox. The length \metropolis@blockwd is set to \paperwidth, but you could choose any other value you like.
• Second, for the placement of the block environment on the slide: You might want to consider using the t option on the frame. Enabling it, elements on the slide are not vertically centered but placed starting at the top of the slide. I feel it's easier this way to reach proper placement of the block by using \vpsace*{2cm} (with any value you like).

Here's the code:

\documentclass{beamer}
\usetheme{metropolis}

\makeatletter
\newlength{\metropolis@blockwd}
\setlength{\metropolis@blockwd}{\paperwidth}
\renewcommand{\metropolis@block}[1]{
  \par\vskip\medskipamount%
  \setlength{\parskip}{0pt}
  \ifbeamercolorempty[bg]{block title#1}{%
    \begin{beamercolorbox}[rightskip=0pt plus 4em,wd=\metropolis@blockwd]{block title#1}}{%
  \ifbeamercolorempty[bg]{block title}{%
    \begin{beamercolorbox}[rightskip=0pt plus 4em,wd=\metropolis@blockwd]{block title#1}%
  }%
  {%
    \begin{beamercolorbox}[
      sep=\dimexpr\metropolis@blocksep-\metropolis@blockadjust\relax,
      leftskip=\metropolis@blockadjust,
      rightskip=\dimexpr\metropolis@blockadjust plus 4em\relax,
      wd=\metropolis@blockwd
    ]{block title#1}%
  }}%
      \usebeamerfont*{block title#1}%
      \metropolis@strut%
      \insertblocktitle%
      \metropolis@strut%
  \end{beamercolorbox}%
  \nointerlineskip%
  \ifbeamercolorempty[bg]{block body#1}{%
    \begin{beamercolorbox}[vmode,wd=\paperwidth]{block body#1}}{
  \ifbeamercolorempty[bg]{block body}{%
    \begin{beamercolorbox}[vmode,wd=\paperwidth]{block body#1}%
  }{%
    \begin{beamercolorbox}[sep=\metropolis@blocksep, vmode,wd=\metropolis@blockwd]{block body#1}%
    \vspace{-\metropolis@parskip}
  }}%
      \usebeamerfont{block body#1}%
      \setlength{\parskip}{\metropolis@parskip}%
}
\makeatother

\begin{document}

\begin{frame}[t]{Frame}
\centering
\vspace*{2cm}
\metroset{block=fill}
\begin{block}{\vspace*{-3ex}}
\centering
A wide block
\end{block}
\end{frame}

\end{document}

You can use one column that spans the whole \paperwidth. For vertical alignment, as in the other answers, use the option [t] for the frame to top align, and then push down with e.g. \vspace*{}.

\documentclass{beamer}
\usetheme{metropolis}
\begin{document}

\begin{frame}[t]{Frame}
  \begin{columns}
    \begin{column}{\paperwidth}
      \vspace*{3\baselineskip}
      \metroset{block=fill}
      \begin{block}{\vspace*{-3ex}}
        \centering
        A wide block
      \end{block}
    \end{column}
  \end{columns}
\end{frame}

\end{document}