documentclass[10pt,aspectratio=169, compress]{beamer}
usetheme[progressbar=frame title, numbering=fraction, titleformat title=
smallcaps]{metropolis}      % Use metropolis theme
setbeamertemplate{section in toc}[sections numbered]
setbeamertemplate{subsection in toc}[subsections numbered]
useoutertheme[subsection=false]{miniframes}
setbeamercolor{section in head/foot}{fg=white, bg=mDarkTeal}
setbeamerfont{section in head/foot}{series=bfseries}


begin{document}
section{Section I}
subsection*{Section I}
begin{frame}{Introduction}
end{frame}


section{Section II}
subsection*{Section II}
begin{frame}{Frame 2}
Something
end{frame}
end{document}

documentclass[10pt,aspectratio=169, compress]{beamer}
usetheme[progressbar=frame title, numbering=fraction, titleformat title=
smallcaps]{metropolis}      % Use metropolis theme
setbeamertemplate{section in toc}[sections numbered]
setbeamertemplate{subsection in toc}[subsections numbered]
useoutertheme[subsection=false]{miniframes}
setbeamercolor{section in head/foot}{fg=white, bg=mDarkTeal}
setbeamerfont{section in head/foot}{series=bfseries}

makeatletter
setbeamertemplate{frametitle}{%
  nointerlineskip%
  begin{beamercolorbox}[%
      wd=paperwidth,%
      sep=0pt,%
      leftskip=metropolis@frametitle@padding,%
      rightskip=metropolis@frametitle@padding,%
      ht=2.25ex,%%%%%% NEW !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
      dp=1.1ex, %%%%%% NEW !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    ]{frametitle}%
  metropolis@frametitlestrut@start%
  insertframetitle%
  nolinebreak%
  metropolis@frametitlestrut@end%
  end{beamercolorbox}%
}
makeatother

begin{document}
section{Section I}
subsection{Section I}
begin{frame}{Introduction}
end{frame}


section{Section II}
subsection*{Section II}
begin{frame}{Frame 2}
Something
end{frame}
end{document}