以agent为视角，分passive 和active 工作模式
net.if.in[if,<mode>]
If: 接口， eth0
mode: bytes, packets, errors, dropped
net.if.out[if,<mode>] net.if.total[if,<mode>]
net.tcp.listen[port] net.tcp.port[<ip>,port] net.tcp.service[service,<ip>,<port>] net.udp.listen[port]
kernel.maxfiles kernel.maxproc
system.cpu.intr system.cpu.load[<cpu>,<mode>] system.cpu.num[<type>] system.cpu.switches system.cpu.util[<cpu>,<type>,<mode>]
vfs.dev.read[<device>,<type>,<mode>] vfs.dev.write[<device>,<type>,<mode>] vfs.fs.inode[fs,<mode>]