ena: Support srd metrics with sysctl

This commit introduces SRD metrics through sysctl.
The metrics can be queried using the following sysctl node:
sysctl dev.ena.<device index>.ena_srd_info

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

(cherry picked from commit 36d42c862f4a5643f6e2395e8d7b7e5c4580499a)