The "activity" trigger was inspired by the heartbeat one, but aims at
providing instant indication of the immediate CPU usage. Under idle
condition, it flashes 10ms every second. At 100% usage, it flashes
90ms every 100ms. The blinking frequency increases from 1 to 10 Hz
until either the load is high enough to saturate one CPU core or 50%
load is reached on a single-core system. Then past this point only the
duty cycle increases from 10 to 90%.

This results in a very visible activity reporting allowing one to
immediately tell whether a machine is under load or not, making it
quite suitable to be used in clusters.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>