AsyncState breaks when used in multiple service collections

[xakep139]

Description

Current implementation of AsyncState has both its pool and AsyncLocal as static fields, but the counter isn't static:

extensions/src/Libraries/Microsoft.Extensions.AsyncState/AsyncState.cs

Line 17 in a8e1751

private int _contextCount;

This counter is used to generate indexes of IAsyncContexts, thus when used within different service collections, it start to pointing to items that were added previously (and they have different types potentially).

Reproduction Steps

using Microsoft.Extensions.AsyncState;
using Microsoft.Extensions.DependencyInjection;

await using var spOne = PrepareAsyncState(new Tuple<double>(3.14));
await using var spTwo = PrepareAsyncState(new Tuple<int>(42));

// This line throws:
_ = spOne.GetRequiredService<IAsyncContext<Tuple<double>>>().Get();

static ServiceProvider PrepareAsyncState<T>(T value)
	where T : notnull
{
	var services = new ServiceCollection().AddAsyncState().BuildServiceProvider();
	services.GetRequiredService<IAsyncState>().Initialize();
	services.GetRequiredService<IAsyncContext<T>>().Set(value);
	return services;
}

Expected behavior

Code doesn't throw.

Actual behavior

The code throws InvalidCastException.

Regression?

No

Known Workarounds

No response

Configuration

No response

Other information

Related discussion exists regarding the IHttpContextAccessor: aspnet/HttpAbstractions#753

[xakep139]

cc @davidfowl @andrey-noskov @geeknoid @galakt