As part of open-telemetry/opentelemetry-specification#238 we should not set the newly created Span as the current instance for Tracer.start_span().

As part of this, we could also add a Tracer.start_as_current_span() (or with a similar name) method that would essentially do:

def start_as_current_span(self, ..., end_on_exit): # Guess end_on_exit should be optional as well
    span = self.start_span(...)
    return self.use_span(span, end_on_exit)

cc @Oberon00 (whose latest PR I sadly missed - sorry ;( )