The sflow module samples requests and sends them to an sFlow collector as
UDP datagrams, along with counters for the server as a whole.  The
collector's address and the sampling rate are read from a configuration
file, the one host-sflow's hsflowd writes, and the rate travels in every
datagram so that a collector can scale the samples back up.

This port only installs the module sources.  The module itself is built by
the nginx ports when their SFLOW option is enabled.