Start a new Tracer object, and store it in self.tracers.

def _start_tracer(self, origin):
        """
        Start a new Tracer object, and store it in self.tracers.
        """
        tracer = self._tracer_class(log=self.log)
        tracer.data = self.data
        fn = tracer.start(origin)
        self.tracers.append(tracer)
        return fn