In General, the main problem I have is this monitoring of the channel load on the web server with the division into domains. Maybe there will be tips on measurement algorithms?
I understand correctly, your web server serves multiple hostnames and you want to know the statistics of the requests for domain names? In this case, I guess, like SNMP and netflow will help you is unlikely. The fact that in the General case, these queries differ in the header "Host:" HTTP-messages. I doubt that the SNMP agent of your router will provide this information. Netflow in these parameters is also not taken into account. If I understand correctly your task, it is more logical to parse the access logs. The obtained data can be sent to the collectd/graphite, for example.