Exception when disposing BetterStackLoggerProvider

[Jericho]

I am referencing Formitable.BetterStack.Logger.Microsoft v0.1.2 and I get the following exception when my app is terminating:

System.AggregateException
  HResult=0x80131500
  Message=One or more errors occurred. (The CancellationTokenSource has been disposed.)
  Source=System.Private.CoreLib
  StackTrace:
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait(TimeSpan timeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait(TimeSpan timeout)
   at Formitable.BetterStack.Logger.Microsoft.BetterStackLoggerProvider.Dispose()
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.Dispose()
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.Dispose()
   at Picton.IntegrationTests.Program.<Main>d__0.MoveNext() in D:\_build\Picton\Source\Picton.IntegrationTests\Program.cs:line 25
   at Picton.IntegrationTests.Program.<Main>d__0.MoveNext() in D:\_build\Picton\Source\Picton.IntegrationTests\Program.cs:line 26
   at Picton.IntegrationTests.Program.<Main>()

  This exception was originally thrown at this call stack:
    [External Code]

Inner Exception 1:
ObjectDisposedException: The CancellationTokenSource has been disposed.

This is due to the fact that the cancellation token is disposed in the Dispose method like so:

_cancellationTokenSource.Dispose();

meanwhile the _flushTask is still running and one of the lines in this flush task is the following:

while (!_cancellationTokenSource.IsCancellationRequested)

This line causes the exception because the cancellation token has been disposed.

I propose the following changes in the Dispose method to resolve this problem:

1. Invoke Cancel() on the cancellation token source rather than disposing it. This ensures that no exception is thrown in "_flushTask" and _cancellationTokenSource.IsCancellationRequested can be evaluated without problem. It will return true, causing the task to stop gracefully which is the desired behavior.
2. There's a possibility that a few messages might still be in the queue when "_flushTask" is stopped. Therefore the try ... catch ... code in Dispose needs to be altered to process these remaining messages.
3. Add a finally section to the try ... catch ... where the cancellation token source can be safely disposed.

I will submit a PR where I implement the changes I propose.

For reference, this is what my app looks like:

namespace Picton.IntegrationTests
{
	public class Program
	{
		public static async Task Main()
		{
			var cts = new CancellationTokenSource();
			Console.CancelKeyPress += (s, e) =>
			{
				e.Cancel = true;
				cts.Cancel();
			};

			var services = new ServiceCollection();
			ConfigureServices(services);
			using var serviceProvider = services.BuildServiceProvider();
			var app = serviceProvider.GetService<IHostedService>();
			await app.StartAsync(cts.Token).ConfigureAwait(false);
		}

		private static void ConfigureServices(ServiceCollection services)
		{
			services.AddHostedService<TestsRunner>();

			services
				.AddLogging(logging =>
				{
					var betterStackToken = Environment.GetEnvironmentVariable("BETTERSTACK_TOKEN");
					if (!string.IsNullOrEmpty(betterStackToken))
					{
						logging.AddBetterStackLogger(options =>
						{
							options.SourceToken = betterStackToken;
						});
					}

					logging.AddSimpleConsole(options =>
					{
						options.SingleLine = true;
						options.TimestampFormat = "yyyy-MM-dd HH:mm:ss ";
					});

					logging.AddFilter("*", LogLevel.Debug);
				});
		}
	}
}