update prometheus.client to v6 (#90)

Author: renovate[bot]

src/Prometheus.Client.HttpClient/Prometheus.Client.HttpClient.csproj

@@ -3,7 +3,7 @@
     <TargetFramework>net6.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Prometheus.Client" Version="5.2.0" />
+    <PackageReference Include="Prometheus.Client" Version="6.0.0" />
     <PackageReference Include="Microsoft.Extensions.Http" Version="6.0.1" />
   </ItemGroup>
   <ItemGroup>