ena: Support customer metric with sysctl

This commit adds sysctl support for customer metrics.
Different customer metrics can be found in the following sysctl node:
sysctl dev.ena.<device index>.customer_metrics

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.

(cherry picked from commit f97993ad7b9c9e4787bd198d11f348c271af513e)