Add ENI metrics for the ENA driver

The new HAL allows the driver to read extra ENI stats. Exact meaning of
each of them can be found in base/ena_defs/ena_admin_defs.h file and
structure ena_admin_eni_stats.

Those stats are being updated inside of the timer service, which is
executed every second.
ENI metrics are turned off by default. They can be enabled, using the
sysctl node: dev.ena.X.eni_metrics.update_delay
0 value in this node means that the update is turned off. Other values
determine how many seconds must pass, before ENI metrics will be
updated.

They can be acquired, using sysctl:

sysctl dev.ena.X.eni_metrics

Where X stands for the interface number.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27118