Record virtual network relevance time in a more precise way:

1. When doing a sync with pre-existing network interfaces at start
up, correctly pick the earliest relevance time rather than the one
associated to the first local attachment to be processed.

2. Add a prometheus gauge to track the delay of the recorded
relevance time wrt the real relevance time.

3. Log a warning when a relevance delay is detected.

Follows a list of changes.

Rename: "creat" -> "create".

Access to Prometheus histograms and counters: from "With(...)" ->
"WithLabelValues(...)".

Refactorings to streamline code and reduce duplication.

Better wording of comments on Prometheus metrics.

Enforce consistent pass order of related function parameters.

Print VNI consistently in hex in logs and error messages.

Get rid of uninteresting Prometheus metrics.