List of "serveradmin command" commands
Depending on how much spelunking you are willing to do you can look at the html command templates in the servermgrd bundles for a partial list of command names/formats. For example to get a list of commands that the afp plugin accepts you could look at:

/Applications/Server.app/Contents/ServerRoot/usr/share/servermgrd/html/servermgr_afp.html


and look at the javascript in the template.

Look at the selection == "command_name" for the names of the commands and the rest of the commandValue for the plist formatted command. Then you can use the serveradmin command format to send the command to the plugin.

A different (and more up to date) way to find out commands is to look at the Server activity log to see the plist formatted commands sent by Server.app to the servermgrd backend. Look in 

~/Library/Logs/Server.log


for the commands and their replies. You may need to turn on the logging from one of the menus (I'm away from my server right now so I'm not sure how I enabled the logging).

If you want network throughput, open Server.app, go to the stats panel, select the network stats and then look at the Server.log to get the commands you need to use.