feat: add metrics for count of known NNC and NCs

[rbtr]

Reason for Change:

Exposes a metric for known NNC and NCs in the NNC reconciler and sync host NC loop.

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• relevant PR labels added

Notes:

[rbtr]

/azp run Azure Container Networking PR

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timraymond]

Typically, don't prometheus metrics that are counts end with _total?

[rbtr]

Yes, but these are not counts in the way that Prometheus uses that term.

They say add _total

for a unit-less accumulating count

This is an instantaneous count and may go up or down, ie not accumulating. I don't think total is appropriate. You wouldn't d/dt (nnc_ncs) to get a rate, for example, but d/dt (http_requests_total) is reasonable and what they intend.

Their general guidance is to suffix the metric with the plural units - NC is the unit here, as is IP in the ones above.