Use new .NET Core 3.0 API to get the total number of allocated bytes for all threads

[adamsitnik]

preview6 of .NET Core 3.0 exposes a new method that allows to get the number of allocated bytes for all threads. By using it we can fix #1153 and #723

Sample:

public class MultiThreadedAllocation
{
    public const int Size = 1_000_000;
    public const int ThreadsCount = 10;

    private Thread[] threads;

    [IterationSetup]
    public void SetupIteration()
    {
        threads = Enumerable.Range(0, ThreadsCount)
            .Select(_ => new Thread(() => GC.KeepAlive(new byte[Size])))
            .ToArray();
    }

    [Benchmark]
    public void Allocate()
    {
        foreach (var thread in threads)
        {
            thread.Start();
            thread.Join();
        }
    }
}

dotnet run -c Release -f netcoreapp2.1 --filter *MultiThreadedAllocation* --runtimes netcoreapp3.0 netcoreapp2.1 net461 --memory 

Method	Runtime	Toolchain	Gen 0	Gen 1	Gen 2	Allocated
Allocate	Clr	net461	2000.0000	2000.0000	2000.0000	10024816 B
Allocate	Core	netcoreapp2.1	2000.0000	2000.0000	2000.0000	-
Allocate	Core	netcoreapp3.0	2000.0000	2000.0000	2000.0000	10001360 B

[adamsitnik]

/cc @stephentoub

[stephentoub]

Thanks!

[AndreyAkinshin]

@adamsitnik 👍