ActivityProcessor should implement IAsyncDisposable for netstandard2.1?

[eddynaka]

After the PR #975, ActivityProcessor will implement IDisposable. With netstandard2.1, we can also implement IAsyncDisposable.

Below one example that @pcwiese sent me:

public class TestClass : IDisposable
#if NETSTANDARD2_1
, IAsyncDisposable
#endif
{
/// <inheritdoc/>
    public void Dispose()
    {
        if (Interlocked.CompareExchange(ref this.disposed, 1, 0) == 1)
        {
            return;
        }

        this.Dispose(true);
        GC.SuppressFinalize(this);
    }

#if NETSTANDARD2_1
    /// <inheritdoc/>
    public async ValueTask DisposeAsync()
    {
        if (Interlocked.CompareExchange(ref this.disposed, 1, 0) == 1)
        {
            return;
        }

        // see https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync#the-disposeasync-method
        // Perform async cleanup.
        await this.DisposeAsyncCore().ConfigureAwait(false);

        // Dispose of managed resources.
        this.Dispose(false);

        // Suppress finalization.
        GC.SuppressFinalize(this);
    }
#endif

    /// <summary>
    /// Releases unmanaged and - optionally - managed resources.
    /// </summary>
    /// <param name="disposing"><c>true</c> to release both managed and unmanaged resources; <c>false</c> to release only unmanaged resources.</param>
    protected virtual void Dispose(bool disposing)
    {
    }

#if NETSTANDARD2_1
    /// <summary>
    /// Releases unmanaged and - optionally - managed resources asynchronously.
    /// </summary>
    /// <returns>a ValueTask</returns>
    protected virtual ValueTask DisposeAsyncCore()
    {
        return default;
    }
#endif
}

cc: @reyang

[martinjt]

Doesn't seem like there is demand for this, so I'm going to close, feel free to reopen if you think this is still required.