"Can not start an Activity that was already started" in BaseProducer

[gius]

Describe the bug
When producing an event, System.InvalidOperationException: '"Can not start an Activity that was already started"' is thrown (and immediately swallowed) by System.Diagnostics.DiagnosticSource.dll!System.Diagnostics.Activity.NotifyError(System.Exception exception).

The reason is that in BaseProducer.Produce, we explicitly call act?.Start(), but the activity has already been started by the previous line's call to ProducerActivity.Start.

eventuous/src/Core/src/Eventuous.Producers/BaseProducer.cs

Lines 46 to 47 in b72c9c0

	var (act, producedMessage) = ProducerActivity.Start(messagesArray[0], DefaultTags);
	return (act?.Start(), new[] { producedMessage });

To Reproduce
Steps to reproduce the behavior:

1. Enable Break When Thrown for System.InvalidOperationException in Visual Studio's Exception Settings
2. Publish an event through RabbitMqProducer
3. See error

Expected behavior
No exception should be thrown.

I can provide a PR if you want. As far as I can understand, simply changing

eventuous/src/Core/src/Eventuous.Producers/BaseProducer.cs

Line 47 in b72c9c0

return (act?.Start(), new[] { producedMessage });

to return (act, new[] { producedMessage }); should be enough.