Is your feature request related to a problem?
Custom implementation of Session cannot be provided through the constructor of the different exporters. This might be needed if the zipkin server is secured with OAuth2 and we need to send a token in the request.

Describe the solution you'd like
Exporters constructor should have an optional parameter session:Session|None that allow the user to give its implementation of Session. The Exporter session would fallback to the current Session if the parameter is not used.

Describe alternatives you've considered
The session attribute can be modified after initialization but it doesn't respect dependency injection principle.