图中阴影部分要怎么画？

求助大家图中阴影部分要怎么画？

这是源代码

\documentclass[12pt]{article}
\pagestyle{empty}
\usepackage{amsfonts,amssymb,amsmath,amsthm,color,colortbl,graphicx}
\usepackage{tikz}
\usepackage[Symbol]{upgreek}
\usepackage{stmaryrd}
\usepackage{caption}
\usepackage{tikz}
\usepackage{float}
\usepackage{tikz,amsmath,amssymb}
\usetikzlibrary{shapes.misc,shapes.geometric,shapes.symbols}
\usetikzlibrary{shapes.geometric, arrows}
\usepackage{tikz,amsmath,amssymb}
\usetikzlibrary{shapes.misc,shapes.geometric,shapes.symbols,arrows.meta}
%\usepackage{refcheck}
\setlength{\oddsidemargin}{0cm} \setlength{\evensidemargin}{0cm}
\setlength{\topmargin}{-1cm} \setlength{\footskip}{1cm}
\setlength{\textheight}{22.0cm} \setlength{\textwidth}{16.5cm}
\numberwithin{equation}{section}
\newtheorem{lemma}{Lemma}[section]
\newtheorem{theorem}{Theorem}[section]
\newtheorem{proposition}{Proposition}[section]
\newtheorem{corollary}{Corollary}[section]
\theoremstyle{definition}
\newtheorem{definition}{Definition}[section]
\theoremstyle{remark}
\newtheorem{remark}{\textbf{Remark}}[section]
\newcommand{\abs}[1]{\left\vert#1\right\vert}
\newcommand{\p}{\partial}
\newcommand{\norm}[1]{\left\Vert#1\right\Vert}
\newcommand{\dd}{\mathrm{d}}
\newcommand{\bfn}{\mathbf{n}}
\newcommand{\Dom}{\mathrm{Dom}}
\newcommand{\Ran}{\mathrm{Ran}}
\newcommand{\sign}{\mathrm{sign}}
%\newcommand{\diam}{\mathrm{diam}}
% Rome Numbers --------------------------------------------------------------
\makeatletter
\newcommand{\rmnum}[1]{\romannumeral #1}
\newcommand{\Rmnum}[1]{\expandafter\@slowromancap\romannumeral#1@}
\makeatother
%%%---------------------------------------------------------------------------------------------------------
%%%---------------------------------------------------------------------------------------------------------
\begin{document}
\begin{figure}[H]\centering
    \begin{tikzpicture}
\begin{scope}[x={(1cm,0cm)},y={(0cm,1cm)}]
\draw[-stealth](-4,0)--(4,0);
\draw[-stealth](0,-3)--(0,3)node[above]{$(z)$};
\draw[-](0,0)--(2.5,3);
\node[left]at(0.13,-0.15){$\scriptstyle O$};
\draw[-](3,-3)--(0,0);
\end{scope}
\draw[-{Stealth[scale=0.5]}](-50:0.24)arc(-50:60:0.24);
\node at(0.45,0.1){$\frac{\pi}{\alpha}$};
\end{tikzpicture}
\end{figure}        
\end{document}

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