If you are using Netstack2 as the system network stack (the current default for every product except those on RISC-V), you can enable tracing for the netstack by setting the enable_netstack2_tracing GN arg to true. If you are using Netstack3, you don't have to do anything to enable tracing: Netstack3 has tracing support compiled in by default.
Ensure that your fx set includes --with-base //bundles/tools, otherwise Netstack will fail to connect to trace_manager at initialization time.
Make sure to rebuild and reboot your emulator or OTA/flash your device to ensure the system picks up the changes.
You can use the ffx trace subcommand (see docs) to run a trace on a running Fuchsia system (e.g. an emulator, smart display, or NUC) that is connected to your development host machine. For example:
ffx trace start --categories net,kernel:sched --duration 10
This will record a trace of all categories in the kernel:sched and net categories. (kernel:sched includes detailed scheduler information, and net includes all events emitted by the netstack.) You might want to record this trace while doing something that exercises the netstack on the Fuchsia device; for example, running iperf3.
When you run ffx trace, it will produce a trace file that is viewable in Perfetto.