event/dsw: add port busy cycles xstats
event/dsw: add port busy cycles xstats

DSW keeps an internal port load estimate, used by the load balancing
mechanism. As a side effect, it keeps track of the total number of
busy cycles since startup. This metric is indirectly exposed in the
form of DSW xstats' "port_<n>_event_proc_latency", which is the total
number of busy cycles divided by the total number of events processed
on a particular port.

An external application can take (event_latency * dequeued) to go back
to busy_cycles. One reason for doing this is to measure the port's
load during a longer time period, without resorting to sampling
"port_<n>_load". However, as the number dequeued events grows, a
rounding error in event_latency renders the application-calculated
busy_cycles inaccurate.

Thus, it makes sense to directly expose the number of busy cycles as a
DSW xstats, even though it might seem redundant.

Signed-off-by: Mattias Rönnblom <ba23a026ae54c0c95a377f3421ce76e4585cf9f8@ericsson.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk