Shaderlab metallic map

How to use a metallic texture instead of the metallic and smoothness sliders? this is my code:

 Shader "Stencil/StencilObject" {
     Properties{
         _Color("Color", Color) = (1,1,1,1)
         _MainTex("Albedo (RGB)", 2D) = "white" {}
         _Glossiness("Smoothness", Range(0,1)) = 0.5
         _Metallic("Metallic", Range(0,1)) = 0.0
         _BumpMap("Normal Map", 2D) = "bump" {}
     }
         SubShader{
             Tags { "RenderType" = "Opaque" "Queue" = "Geometry" }
             LOD 200
 
             Stencil {
                 Ref 1
                 Comp notequal
                 Pass keep
             }
             CGPROGRAM
         // Physically based Standard lighting model, and enable shadows on all light types
         #pragma surface surf Standard fullforwardshadows
 
         // Use shader model 3.0 target, to get nicer looking lighting
         #pragma target 3.0
 
         sampler2D _MainTex;
         sampler2D _BumpMap;
         struct Input {
             float2 uv_MainTex;
             float2 uv_BumpMap;
         };
 
         half _Glossiness;
         half _Metallic;
         fixed4 _Color;
 
         void surf(Input IN, inout SurfaceOutputStandard o) {
             // Albedo comes from a texture tinted by color
             fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
             o.Albedo = c.rgb;
             o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
             o.Metallic = _Metallic;
             o.Smoothness = _Glossiness;
             o.Alpha = c.a;
         }
         ENDCG
     }
         FallBack "Diffuse"
 }