Find container network metrics by looking at /proc/$PID/net/dev of the container process.

def _report_net_metrics(self, container, tags):
        """Find container network metrics by looking at /proc/$PID/net/dev of the container process."""
        if self._disable_net_metrics:
            self.log.debug("Network metrics are disabled. Skipping")
            return

        proc_net_file = os.path.join(container['_proc_root'], 'net/dev')

        try:
            if container['Id'] in self.network_mappings:
                networks = self.network_mappings[container['Id']]
            else:
                networks = self.docker_util.get_container_network_mapping(container)
                if not networks:
                    networks = {'eth0': 'bridge'}
                self.network_mappings[container['Id']] = networks
        except Exception as e:
            # Revert to previous behaviour if the method is missing or failing
            # Debug message will only appear once per container, then the cache is used
            self.log.debug("Failed to build docker network mapping, using failsafe. Exception: {0}".format(e))
            networks = {'eth0': 'bridge'}
            self.network_mappings[container['Id']] = networks

        try:
            with open(proc_net_file, 'r') as fp:
                lines = fp.readlines()
                """Two first lines are headers:
                Inter-|   Receive                                                |  Transmit
                 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
                """
                for l in lines[2:]:
                    cols = l.split(':', 1)
                    interface_name = str(cols[0]).strip()
                    if interface_name in networks:
                        net_tags = tags + ['docker_network:'+networks[interface_name]]
                        x = cols[1].split()
                        m_func = FUNC_MAP[RATE][self.use_histogram]
                        m_func(self, "docker.net.bytes_rcvd", long(x[0]), net_tags)
                        m_func(self, "docker.net.bytes_sent", long(x[8]), net_tags)

        except IOError as e:
            # It is possible that the container got stopped between the API call and now
            self.log.debug("Cannot read network interface file, container likely raced to finish : {0}".format(e))