Accessing the properties of the IApiDescriptionGroupCollectionProvider is blocking threads and memory leaks.

Test environment:
```
>dotnet --info
.NET Core SDK (reflecting any global.json):
Version: 3.1.100
Commit: cd82f021f4

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.100\

Host (useful for support):
Version: 3.1.0
Commit: 65f04fb6db

.NET Core SDKs installed:
3.0.100 [C:\Program Files\dotnet\sdk]
3.0.101 [C:\Program Files\dotnet\sdk]
3.1.100 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.App 3.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
```
Target framework is **netcoreapp3.1**

The code to reproduce the problem is very simple.

```
public class MyServiceConvention : IApplicationModelConvention
{
public void Apply(ApplicationModel application)
{
foreach (var controller in application.Controllers)
{
if (controller.ApiExplorer.IsVisible == null)
{
controller.ApiExplorer.IsVisible = true;
}

foreach (var action in controller.Actions)
{
if (action.ApiExplorer.IsVisible == null)
{
action.ApiExplorer.IsVisible = true;
}
}
}
}
}

public class Startup
{
public Startup(IConfiguration configuration)
{
Configuration = configuration;
}

public IConfiguration Configuration { get; }

public void ConfigureServices(IServiceCollection services)
{
services.AddControllersWithViews();

services.Configure(x =>
{
x.Conventions.Add(new MyServiceConvention());
});
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Home/Error");
}

app.UseStaticFiles();

app.UseRouting();

app.UseAuthorization();

app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
"default",
"{controller=Home}/{action=Index}/{id?}");
});
}
}

```

Get **/api/home** blocking and memory leaks.
```
[Route("api/[controller]")]
public class HomeController : Controller
{
private readonly IApiDescriptionGroupCollectionProvider _descriptionProvider;

public HomeController(IApiDescriptionGroupCollectionProvider descriptionProvider)
{
_descriptionProvider = descriptionProvider;
}

[HttpGet]
public void Index()
{
var items = _descriptionProvider.ApiDescriptionGroups.Items; // blocking and memory leaks.
}

[HttpPost]
public void Privacy(Type type)
{

}
}
```


**If I remove the Type parameter of the Provacy method, the above problem does not occur.**