configuration option to not create span if can not extract from headers

[andrascz]

When using Fiber in a microservice environment it is beneficial if root spans are not created on the service level as the trace sampling is done at the ingress level. So in this case it is counter intuitive to create a new span if there is none in the request.
There is a workaround with the Filter function, but it would be nice to be able to configure this, to not work twice on the headers.