Return the throughput of the specified instance.

        :param instId: the id of the protocol instance

def getThroughput(self, instId: int) -> float:
        """
        Return the throughput of the specified instance.

        :param instId: the id of the protocol instance
        """
        # We are using the instanceStarted time in the denominator instead of
        # a time interval. This is alright for now as all the instances on a
        # node are started at almost the same time.
        if instId not in self.instances.ids:
            return None
        perf_time = time.perf_counter()
        throughput = self.throughputs[instId].get_throughput(perf_time)
        return throughput